Showing posts with label Plant. Show all posts
Showing posts with label Plant. Show all posts

Important Facts about Red Algae

 
Rhodophyta species, which owe their typically reddish color to a phycobilin pigment, possess no flagellated motile cells. Nearly all red algae are marine, although a few genera (for example, Batrachospermum) inhabit swiftly running freshwater. Some simple types are unicellular, but most have a basically filamentous organization, which often is elaborated into compound body structures that frequently show branching of great regularity. In the higher forms there are obvious pit connections between adjacent cells of the same filament.


Sexual reproduction is highly developed and normally exceedingly complex in red algae. It is based on the fertilization of egg cells by small, nonmotile, male gametes (spermatia). In many cases it involves the transfer of fertilized nuclei into specialized auxiliary cells and filaments (gonimoblasts) prior to their inclusion in spores. Alternation of cytologically different generations (usually isomorphic or identical in structure) is found in the more highly evolved genera. The reserve foodstuff is a polysaccharide known as Floridean starch.

The single class Rhodophyceae is divided into two subclasses, Bangiophycidae and Florideophycidae. Members of the former are more primitive and lack the complicated postfertilization processes found in those of the latter. They are also less highly organized morphologically. A well-known representative is laver (Porphyra).

Florideophycidae's orders—which include Ceramiales, Compsopogonales, Corallinales, Cryptonemiales, Gigartinales, Nemaliales, and Rhodymeniales—have been established based on features of the life cycle and postfertilization development of their species. The typical life cycle of one of the higher Florideophycean red algae, such as Polysiphonia, may be briefly summarized as follows.

The first generation, the gametophytes, produces spermatia and egg cells, commonly on different individuals. From the fertilized egg a small organism called the carposporophyte develops parasitically on the female gametophyte. It is diploid (having a double number of chromosomes in its nuclei) and consists of only a few rows of cells or filaments. The carposporophyte gives rise to a cluster of spores, usually inside a protective sheath (cystocarp). These spores are released and germinate to give rise to the second (ordinarily isomorphic) generation of organisms, which, unlike the gametophytes, has diploid nuclei. The number of chromosomes is reduced by half in the developing tetraspores (spores that develop in structures called tetrasporangia, which form on the second-generation algae). On being liberated, the tetraspores germinate to produce a new generation of gametophytes. Thus the life cycle is completed.

Certain genera of the order Corallinales deposit lime in the thallus, making the body rigid and stony. The body can be either solid (as in Lithothamnium) or in articulated segments (as in Corallina).

Some of the better-known representatives of the subclass Florideophycidae are Chondrus (Irish moss), Rhodymenia (dulse), and Ceramium (lobster claws). All are common on northern shores.

Some Important Things You Should Know about Green Algae

 
Chlorophyta is a large and important division, and its members show great diversity in structure and reproduction. Species range from microscopic unicellular forms to structurally complex organisms of moderate size. Practically every possible type of cellular organization is found in the group, including unicellular, colonial, filamentous, leafy, and tubular (siphonaceous). Motility by flagella (usually two, rarely four or more) is widespread, especially in the case of the reproductive cells (zoospores and zoogametes).


Owing to the presence of unmasked chlorophylls a and b, the species are grassy green, their pigmentation also being characteristic in green plants (mosses, ferns, and flowering plants). Starch is produced through assimilation. Some forms are symbiotic with fungi to form lichens, and symbiosis with marine invertebrate animals is also known. Sexual reproduction shows a wide range of variation, from fusion of similar motile gametes (isogamy) to that of dissimilar gametes (anisogamy or oogamy) or the fusion of the entire protoplasm content of different cells, as in the filamentous freshwater genus Spirogyra. Chlorophyceae (to which Spirogyra belongs) is the largest class within Chlorophyta, and its species vary the most in form.

Flagellate unicellular forms of the Chlorophyceae are placed in the order Volvocales, the representatives of which may be either single motile cells (such as Chlamydomonas) or colonies of cells joined together. Volvox is the best-known representative of the latter type, its colonies taking the shape of a hollow, slowly revolving sphere just visible to the naked eye. Nonflagellate unicellular or colonial algae compose the order Chlorococcales, of which Trebouxia, the algal symbiont most commonly associated with lichens, is an example.

Filamentous and membranous organization exists in the orders Ulotrichales and Ulvales, respectively. Ulothrix consists of simple unbranched filaments, each cell with a single chromatophore applied to the inner cell wall. Ulva (sea lettuce) forms leafy, platelike thalli two cell layers thick. Coenocytic structure, consisting of a multinucleate mass of protoplasm, is found in the orders Cladophorales, Siphonocladales, Dasycladales, Bryopsidales, and Caulerpales.

The desmids (order Zygnematales) are microscopic unicellular forms with minutely perforated walls. The cells are constricted in the middle and show bilateral to almost radial symmetry. They inhabit freshwater.

Golden Algae, Yellow-Green Algae, and Diatoms

 
Members of Chrysophyta, Xanthophyta, and Bacillariophyta range in size from microscopic, unicellular organisms to relatively small but visible, filamentous types. Although species in each division vary widely from those of the other divisions in form and structure, the groups are united by certain fundamental characteristics. (In fact, some sources do not recognize three separate divisions but instead classify yellow-green algae and diatoms under Chrysophyta.)


Common traits include the presence (in addition to chlorophyll of the a-type) of specific carotenoid accessory pigments (carotenes and xanthophylls), which may or may not mask the green color of chlorophyll a, and the elaboration of oil or a peculiar carbohydrate, chrysose (leucosin), as a product of assimilation (in which nutrients are either transformed or incorporated into protoplasm). The pigments are localized in special structures called chromatophores. Impregnation of the cell wall with silica is widespread in the group, and movement by means of flagella (whiplike processes) is common in many of the unicellular forms and in the sexual reproductive cells. The divisions are distinguished by the following characteristics:

Chrysophyta - chromatophores are golden yellow to brown; unicellular to filamentous
Xanthophyta - chromatophores are green or greenish; unicellular to filamentous
Bacillariophyta - chromatophores are brown; unicellular with an outer sheath, or frustule, of silica.

Chrysophyta species typically move by means of two flagella of unequal length. Many formerly were regarded as protozoa. However, some types are nonmotile, and others show creeping amoeboid movement by means of protoplasmic extrusions. Most of the forms are freshwater.

Certain forms of Xanthophyta use two unequal flagella for motility; others use creeping amoeboid movements. Some (such as those of the genus Vaucheria) form a tubular, unpartitioned, thallus (body), producing multinucleate and multiflagellate zoospores. Xanthophyta contains mainly freshwater and terrestrial species.

Bacillariophyta is peculiar in the form of the silica sheath, or frustule, that it possesses. This consists of two distinct, overlapping halves and is ornamented with symmetrically arranged pores that have a complex ultramicroscopic structure. The symmetry may be radial (around a central axis), as in the order Centrales, or bilateral (right and left sides as mirror images), as in the order Pennales. Free-floating diatoms are an important type of marine and freshwater plankton. Some species form colonies by aggregation of the individual cells.

Introduction to Algal Classification

 
Algae are typically assigned to the kingdom Protista, which also includes protozoa and funguslike organisms (although not the fungi themselves). While that convention will be followed in this article, some experts instead classify algae as plants—kingdom Plantae.


Within Protista, algae are separated into the following taxonomic divisions: golden algae (Chrysophyta), yellow-green algae (Xanthophyta), diatoms (Bacillariophyta), brown algae (Phaeophyta), dinoflagellate algae (Pyrrophycophyta), cryptomonad algae (Cryptophyta), euglenoid algae (Euglenophyta), stoneworts (Charophyta), green algae (Chlorophyta), and red algae (Rhodophyta).

As some of these names indicate, pigmentation plays an important role in algal classification, and its correlation with distinctive structural and reproductive features shows that it is a fundamental trait. In most cases the pigmentation referred to is an accessory one that occurs in addition to the green photosynthetic pigment, chlorophyll, which the accessory pigment frequently masks; thus the externally visible color in some of the groups is not green. In at least two of the divisions (brown algae and red algae), there is strong evidence that the accessory pigment is functional in photosynthetic metabolism, absorbing light energy and transferring it to the chlorophyll.

All algae contain the form of chlorophyll known as chlorophyll a. However, other varieties of the compound may be present as well, depending on the species.

The following article does not discuss blue-green algae. Despite their name, members of this group, which make up the phylum Cyanophycota, are not true algae but are instead a form of bacteria.

Reborn from the Ashes: The Promise of Smoke-Induced Seed Germination

 ,
An old truism teaches us that "where there's smoke, there's fire," but recent findings by Australian botanists are helping us understand why for certain plants, where there is smoke, there is also rebirth. Scientists have known for years that certain species of plants can be coaxed into accelerating their reproductive cycle by exposing them to certain types of smoke--an adaptation that these plants evolved as a means of surviving wildfires; but the discovery of a chemical called butenolide may be the missing link in exploiting this behavior without ever invoking combustion.


Smoke-induced germination is a well-established practice in commercial horticulture. Exposing the seeds of many plant varieties to controlled doses of smoke-either by directly immersing them in smoke or by submerging them in smoke-infused water-can invoke group germination, meaning that an entire crop or nursery batch will grow and flower in relative unison. Plants native to Mediterranean-type climatic regions, such as the southern California chaparral and large swaths of the Australian bush, most commonly respond to smoke-induced germination. These areas are known for their extended low-rainfall seasons that create dry, fire-prone underbrush, forcing the native flora to evolve fire-adapted reproductive countermeasures.

In some respects wildfire can be viewed as an opportunity for these plants to exploit catastrophe. The competitive environment is effectively "reset" by wildfire; access to available growing areas is wiped clean, and contact with groundwater and open exposure to visible sunlight is now up for grabs. Plants primed to germinate after a fire has died out to a smoking smolder are often best-suited to thrive in this reset post-wildfire landscape. Yet, for all the promise of this trait, smoke-induced germination also holds the potential for dire environmental consequences.

One of the primary chemical components of the smoke produced by burning wood is nitrogen dioxide, and initial insights suggested that nitrogen dioxide is one of the principal chemical triggers for smoke-induced seed germination. This discovery worried many environmentalists, however, because nitrogen dioxide is also a common component of smog. If smog forces plant seeds to germinate too soon, when an area has not been cleared by fire, the seeds are effectively wasted on an overly competitive growing environment. Could urban air pollution could be overstimulating and thereby disrupting the natural reproductive cycles of exposed plants? Scientists have a compelling incentive to answer these questions: If they can isolate and identify every component in wildfire smoke that induces seed germination, they could use that knowledge hopefully to counteract the effects of smog.

After 11 years of research, scientists at the University of Western Australia have presented convincing evidence that they have found the primary trigger for many species of smoke-sensitive plants: butenolide, a heterocyclic compound produced by burning plants and wood. What makes this discovery so intriguing is that butenolide exposure induces germination even in plants that are not native to Mediterranean climates, including lettuce, parsley, and the popular herbal remedy Echinacea. This suggests that butenolide-based fertilizers might be used successfully on a staggeringly wide array of commercial plants-even those not commonly preyed upon by wildfires.

This discovery, in turn, could also help control the effects of smog on nitrogen dioxide-sensitive plants. With judicious use of butenolide fertilizer in smog-prone areas, botanists could theoretically encourage mass seed germination during key time periods, compensating for reproductive disruption caused by smog. Thanks to savvy Australian biochemistry, agriculture may soon reap the benefits of wildfire-adapted evolution without ever needing to set even a single plant aflame.

Facts about Commercial Production of Vegetables

 
Market gardens grow relatively small acreages of many seasonal vegetables. They are usually located near populated areas. Before modern refrigeration and rapid transportation, cities were dependent upon these nearby gardens for perishable vegetables. The cities have grown, and suburbs now occupy much of the land formerly occupied by market gardens.


Today the population centers of industrial North America are increasingly supplied with fresh vegetables from the southern and western states, Mexico, and the Caribbean Islands. Similarly, the cities of central and northern Europe receive fresh vegetables from the Mediterranean countries. Due to the perishability of vegetables, their production was formerly limited to about one day's trip from market. However, with the development of modern refrigeration methods and modern transportation, production has spread to more distant areas. This production, often limited to a single crop, is usually called truck farming. Since the term truck farming has various meanings, extensive vegetable production is sometimes specified as vegetable farming.

In adverse climates perishable vegetables of relatively high value, such as tomatoes, cucumbers, lettuce, mushrooms, and rhubarb, are produced in greenhouses, hotbeds, or forcing structures. This is called vegetable forcing. Production in greenhouses (also called hothouses or glasshouses) is common in the colder areas of Europe and the United States. Greenhouse farming may function on a relatively small scale for a local market or on a large scale for distant markets. Low-cost, transparent plastic film used as a glass substitute has stimulated this type of production. 

Vegetable Production in the United States

Vegetable production occurs on farms of various sizes where an assortment of vegetables is the chief or only enterprise, on general farms where a vegetable crop is grown in rotation with agronomic crops, and on large-scale, factory-like farms specializing in one or, at most, a few vegetables. In the United States production on relatively small acreages is concentrated in the states around the Great Lakes; along the Atlantic Seaboard, particularly New Jersey and the Delmarva peninsula; and in the Mississippi Valley. Large-scale, specialized vegetable farms are located in many production areas but are concentrated in the Imperial, San Joaquin, and Salinas valleys of California, the Salt River valley of Arizona, the Rio Grande valley of Texas, and in southern Florida.

Extensive farming of vegetables adapted to storage is also concentrated in favorable growing areas. Potato production in Maine, Long Island (N.Y.), Idaho, and the maritime provinces of Canada and sweet potato production in Louisiana illustrate this practice. Similarly, crops for canning, freezing, pickling, or dehydration are raised in areas favoring dependable quality and production. For this purpose tomatoes, asparagus, and lima beans are grown in California, sweet corn in Wisconsin, green peas in Washington state, snap beans in Oregon, and pickling cucumbers in Michigan. 

Business Aspects of Vegetable Production

Vegetable production involves integrated operations of scheduled plantings, scientific growing, precise pest control, efficient harvesting and handling, quality maintenance after harvest, and orderly marketing. Large growers often handle their own packing and shipping and are referred to as grower-shippers. Valuable land resources, heavy capital investments in equipment, and large seasonal expenditures for supplies, labor, and services are required. A given company may operate in more than one production area in order to reduce overhead costs and maintain its products in the market throughout the year. Although these operations are highly mechanized, large numbers of workers are required, and they are often housed and fed on the ranch and transported to work by bus. Special crews perform specific operations, and communications may be maintained by the use of a two-way radio. Highly specialized operations such as fertilizer application, disease control, and harvesting are often performed under contract by agencies specializing in these services.

Large-scale vegetable farming in areas of favorable climate that are distant from areas of consumption has developed as methods have been devised for producing throughout the year, as the economics of volume production have been realized, and as many operations have been mechanized. This type of farming has also resulted from the spread of supermarkets that require a large and dependable supply of vegetables and, perhaps above all, from the development of rapid, refrigerated transportation to carry the vegetables to the markets. 

Harvesting and Marketing

All vegetables are perishable and deteriorate after harvest unless processed, as by canning or freezing. Ideally, vegetables to be delivered fresh to the market are harvested carefully at desirable maturity, packaged and cooled without delay, and transported and marketed under continuous refrigeration. Many vegetables deteriorate six to eight times as rapidly at room temperature as at 32° F (0° C). Railway cars and trucks refrigerated by ice or by mechanical units rush the vegetables to terminal markets where refrigerated warehouses, refrigerated delivery trucks, refrigerated display cases, and finally, home refrigerators continue to protect the vegetables against unfavorable temperatures. Only the leafy, vegetative commodities and sweet corn should be kept near 32° F in general; the warm-season vegetables should be held at about 50° F (10° C).

Technological developments in packaging and processing coupled with consumer demands for convenience and the widespread use of self-service supermarkets have caused revolutionary changes in vegetable marketing. The total per capita consumption of commercially produced vegetables remains nearly constant, but processed vegetables have increased significantly at the expense of the fresh. For example, fresh lima beans and peas are disappearing from the market. Furthermore, those vegetables marketed fresh are increasingly packaged in preweighed, prepriced, consumer-sized units and in kitchen-ready condition.

Vegetables as Food

 
Vegetables are used almost exclusively for human food in contrast to other groups of agricultural crops, some of which are produced primarily for industrial use, animal feed, or alcoholic beverages. The use of potatoes for alcohol, starch, flour, and animal feed is an exception to this generalization. With certain vegetables such as potatoes, sweet potatoes, garlic, and horseradish the edible portion is also used as the "seed" to propagate the next crop.


Vegetables are included in the first four of the seven basic food groups: 1) leafy, green, and yellow vegetables; 2) citrus fruits, tomatoes, and raw cabbage; 3) potatoes, other vegetables, and fruits; 4) meat, poultry, fish, eggs, nuts, and dried peas and beans; 5) milk and milk products; 6) bread, flour, and cereal grains; 7) butter and fortified margarine.; and Nutritionists recommend the daily consumption of four servings from the first three food groups, including one serving from each of the first two. Once considered to be luxuries because of their high water content and relatively low energy value, many vegetables today are increasingly recognized as sources of vitamins, minerals, and bulk. Additional knowledge of nutrition is expected to increase the per capita consumption of vegetables.

Aside from nutritive value, vegetables are esteemed foods for additional reasons. Because of color, shape, texture, consistency, flavor, and aroma, vegetables appeal to humans' senses of taste, smell, vision, and touch and add to their enjoyment of a varied diet. Flavor and aroma vary from the subtle to the overpowering, with some vegetables prized for their distinctive taste and others for the way in which they combine with or complement other foods.

Carbohydrates

In many temperate-zone diets the potato, sweet potato, and squash are important staples providing carbohydrates, and vegetables high in carbohydrates, such as the cassava, yam, taro, and dasheen, are basic food to millions in the tropics. The carbohydrate content of these vegetables is largely starch, and such crops are known as starchy vegetables.

Another carbohydrate—sugar—makes up from 1% to 15% of some vegetables and contributes greatly to the taste of many fresh vegetables, especially melons and sweet corn.

Protein

Most vegetables are relatively poor sources of proteins. Fresh peas and lima beans, which contain 6% to 8% protein, are exceptions. Dried beans and peas are a more concentrated source of protein than are the succulent vegetables. Vegetarian diets often depend heavily on legumes for protein.
Fats and Oils

Vegetable fats are fats of plant origin, in contrast to animal fats. Most vegetables are extremely low in fats and oils and for this reason are often included in low-fat diets.

Minerals

Although vegetables are relatively low in mineral content, they are consumed in large enough quantities to function as important sources of many minerals. Calcium is in relatively good supply in collards, mustard greens, kale, dandelion greens, and broccoli. It is also present in some other vegetables, such as spinach, chard, and rhubarb, but in these the calcium is not available to the human body. Most vegetables are relatively low in sodium, another important mineral. Beets, carrots, celery, chard, and kale contain more sodium than do other vegetables. Potassium is supplied in part from vegetables. Since the soil is the ultimate source of minerals and trace elements in natural foods, vegetables serve a unique role in that they come from geographically widespread areas and thus tend to minimize the dietary effect of localized soil deficiencies.

Vitamins

Many vegetables have high vitamin contents. All vegetables contain vitamin C in varying amounts. The best sources of this vitamin include tomato, cauliflower, cantaloupe, cabbage, Brussels sprouts, sweet potato, pepper, potato, turnip, and broccoli. Many factors during growth influence vitamin C content, and after harvest, vegetables vary in their ability to retain this vitamin. Since processing and final cooking cause losses, raw vegetables are generally higher in this vitamin than are cooked vegetables.

Yellow and green vegetables contain carotene, which can be converted into vitamin A in the body. Vegetables such as collards, turnip greens, kale, carrot, squash, sweet potato, and cantaloupe in the diet will supply the vitamin A requirement.

The vitamins of the B complex are supplied in part by vegetables. Although vegetables contain relatively low concentrations of thiamine (vitamin B1), they can make a worthwhile overall contribution of this vitamin to the diet. Dry beans and peas are highest in thiamine among the vegetables. Riboflavin (vitamin B2) is present in leafy green vegetables in significant amounts. Another member of the vitamin B complex, niacin, is supplied in significant amounts by asparagus, collards, sweet corn, mushrooms, green peas, and lima beans. Vegetables are one of the best sources of vitamin B6 and are quite high in pantothenic acid, another B vitamin. They are also among the best sources of folic acid. They are not, however, known to furnish any vitamin B12. A number of vegetables contain other members of the B complex.

Green vegetables are good sources of vitamin E, and the green leafy vegetables, tomatoes, and cauliflower are good sources of vitamin K.

The Classification of Vegetables

 
Over 200 kinds of vegetables are known. In the United States there are at least 75 different vegetables, of which about 50 are grown commercially. For convenience, vegetables are grouped in various ways based on such factors as the botanical characteristics of the plant, the part of the plant consumed, the physiological requirements of the crop, methods of culture, perishability, culinary use, food value, color, and season of harvest.


Aside from grouping, a system of classifying and naming is essential to the proper description and identification of a given vegetable crop. Botanically, a vegetable plant is identified by its Latin binomial, giving genus and species, together with botanical variety if needed. It is further identified according to horticultural variety, or cultivar, and often strain. A cultivar includes plants similar in important physical characteristics that are maintained true to type from one population to the next. Strains within varieties differ less significantly and have most of the characteristics of the variety. The designation of a variety of cabbage would be as follows: cabbage (Brassica oleracea var. capitata), horticultural variety Golden Acre, resistant strain.

Botanical and Morphological

Vegetables may be grouped into botanical families on the basis of plant characteristics. They may also be classified morphologically according to the particular part of the plant that is harvested as food. See accompanying tables.

Temperature Requirements

The nonperennial vegetables commonly grown in temperate zones may be separated into cool-season and warm-season vegetables. For most vegetables this grouping parallels the separation into vegetative parts and fruit parts.

In general, cool-season vegetables prefer monthly mean temperatures of 60° to 65° F (15.5°–18° C). They are intolerant of high temperatures but tolerant of some frost. Cool-season crops include cabbage, cauliflower, turnips, radishes, spinach, lettuce, potatoes, celery, and peas. They have a restricted habit of growth and a short growing season. The part eaten is vegetative. It reaches a usable size in a relatively short period. Hence, these crops are grown in most agricultural areas of the world. The vegetable should be stored near 32° F (0° C) after harvest.

In contrast, warm-season crops generally prefer monthly mean temperatures above 65° F (18° C). They are intolerant of low temperatures and of frost. Warm-season crops include tomato, eggplant, okra, pepper, melons, and cucumber. They have a relatively unrestricted habit of growth and require a long growing season. Production is limited to summer in the temperate areas, where it is often desirable to extend the growing season by the use of forcing structures or protective devices. The edible part is the fruit structure. The vegetable should be stored above 50° F (10° C) after harvest.

Cultural Practices

Vegetables may also be classified according to their cultural requirements. In such a system, vegetables may be grouped in the following categories: perennial crops, greens, salad crops, cole crops, root crops, bulb crops, the potato, the sweet potato, beans and peas, solanaceous fruits, cucurbits (vine crops), sweet corn, and okra.