VEGETABLE PHYSIOLOGY. 



'T^HE science which embraces the study and 

 J. investigation of the vegetable kingdom, is 

 known by the name of BOTANY, from the Greek 

 word botane, meaning an herb or grass. That 

 department of the subject which explains the or- 

 ganisation and vital functions of plants, is called 

 Vegetable Physiology; and that which recognises 

 their arrangement into orders, tribes, genera, and 

 species, according to their respective forms and 

 qualities, Systematic Botany. The one relates to 

 functions which are common to all vegetables, the 

 other takes notice only of those structural pecu- 

 liarities which serve to distinguish one species 

 from another, and to enable the botanist to form 

 these into natural and artificial groups. It is to 

 the former of these departments that we now direct 

 attention. 



GENERAL ECONOMY OF VEGETATION. 



Nature and Functions of Plants. The sim- 

 plest forms of life are observable in certain 

 plants and animals, whose economy is limited to 

 the absorption and assimilation of nutriment, and 

 the power of reproduction ; and the difference 

 between these inferior forms is so trifling, that in 

 them the animal and vegetable kingdoms seem to 

 pass into each other. The absolute differences 

 between plants and animals are indeed difficult 

 to define, when they are to be applied to all plants 

 and to all animals. In many cases, form and 

 structure afford no decisive characters whereby we 

 may separate the two kingdoms from each other ; 

 while phenomena usually regarded as pertaining 

 to the animal kingdom are prevalent in the lower 

 forms of plants, and indicate that no reliance can 

 be placed upon their mode of life. In like manner, 

 the chemical distinctions upon which much de- 

 pendence has hitherto been placed, give way before 

 increased knowledge ; for cellulose and starch, 

 long considered as peculiarly vegetable products, 

 are now known to occur in animal structures. 

 The locomotive power of many of the lower algae 

 is greater than that of many animal organisms ; 

 and even the spores, or seeds, of some algae of more 

 complex organisation, move about when freed 

 from their parent, with an activity which appears 

 truly animal, by means of the cilia with which they 

 are provided. When the spore finds a suitable 

 resting-place, its movements cease ; and having 

 thus exchanged its animal-like mode of life for one 

 of a less erratic character, it becomes developed 

 into a beautiful alga in all respects resembling its 

 parent. The two classes, plants and animals, 

 seem, as it were, to start from a common point at 

 the base, the inferior forms bearing a certain simi- 

 larity in structure and functions, which gradually 

 disappears as we ascend in the scale of develop- 

 ment. 



Plants derive their food partly from the soil and 

 partly from the air ; and whatever they take must 

 either be reduced to a liquid or to a gaseous state. 

 The ultimate elements of which plants are com- 

 posed are carbon, oxygen, hydrogen, and nitro- 

 gen. Of these, carbon, which is a solid substance, 

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is the principal ; and as it is insoluble in water, it 

 must be combined with oxygen, so as to form 

 carbonic arid gas, before it can be taken up by- 

 plants. Oxygen is the next in abundance, and it 

 is absorbed principally when combined with nitro- 

 gen, in the form of atmospheric air. Hydrogen is 

 not found in a free state in the atmosphere, and 

 therefore it can only be taken up by plants when 

 combined with oxygen, in the form of water, or 

 with nitrogen, as ammonia, in which last form it 

 exists in animal manure. Nitrogen, though found 

 in very small quantities in plants, is an important 

 element, as it constitutes the principal ingredient 

 in the gluten, which is the most nutritive part of 

 corn and other seeds, and which is essential to the 

 germination and nourishment of young seedling 

 plants. Nitrogen also appears to be a principal 

 agent in the production of colour in leaves and 

 flowers, especially when they first expand. As 

 oxygen is imbibed by plants in combination with 

 all the other elements of which they are composed, 

 it is not surprising that the plant takes up more 

 of this gas than it requires ; and, consequently, it 

 has been furnished with a remarkable apparatus 

 in the leaves, to enable it to decompose the car- 

 bonic acid and other gases which it has absorbed, 

 and to part with the superfluous oxygen. Plants 

 are thus found to improve the air by the removal 

 of carbonic acid, which is injurious to animal 

 life, and by the restoration of oxygen, which is 

 favourable to it ; and so to maintain a neces- 

 sary equilibrium in the atmosphere, as animals 

 are continually absorbing oxygen, and giving out 

 carbonic acid. Light being essential to the 

 decomposition of carbonic acid gas in the leaves, 

 oxygen is not exhaled by plants during the night; 

 but, on the contrary, a small quantity of car- 

 bonic acid gas escapes, and oxygen is absorbed. 

 These processes have been called the respiration 

 of plants; but they are very different from the 

 respiration of animals. 



Development of Vegetable Life. This depends 

 upon the concurrence of certain agents, the prin- 

 cipal of which are heat, air, moisture, light, and 

 soil. No seed can germinate without the concur- 

 rence of the three agents of heat, air, and moisture ; 

 but in the growth of most plants, the agency of 

 soil and light is also necessary. Every perfect 

 seed contains the germ or embryo of a new plant 

 of the same kind as the parent, and a portion of 

 concentrated carbon and nitrogen, in the form of 

 starch and gluten, laid up to serve as nutriment 

 for the young plant, till its organs are sufficiently 

 developed to enable it to seek food for itself. This 

 nutrient matter is either cont?ined in the tissues of 

 the embryo's cotyledons, or laid up beside it in the 

 seed, in the form of separate albumen. The seed 

 is generally furnished with a hardened covering, 

 in order to preserve it in an inert state as long as 

 may be necessary. The common bean will afford 

 a familiar example of the process of germination. 

 As soon as it is put into the ground, it is acted upon 

 by the influence of heat and moisture, which dis- 

 tend its tissues, so as to burst the external integu- 

 ment The agency of the air is next required to 



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