The genera of South African plants : arranged according to the natural system / by William Henry Harvey.

  • William H. Harvey
Date:
1868
Catalogue details

Licence: Public Domain Mark

Credit: The genera of South African plants : arranged according to the natural system / by William Henry Harvey. Source: Wellcome Collection.

    43/556
    unbranched from tho base to the apex, and are connected by cross-bars or imbranchcd veins. Tlie first of these types is generally characteristic of Exogmu, the latter of Endogens; but there are various intermediate conditions, and some Endogens have been called Dictyogens, because they have netted-veined leaves ; several Exogens also have straight-veiued leaves. 171. Leaves usually extend horizontally, and have an upper and an under sm'face, difiering in anatomical structure. In the cellular stratum of the upper surface tho cells are closely set and placed vertically, with their smallest ends next tho surface; in the lower stratum the cells are more or less horizontal, more loosely placed, and very generally have large empty spaces between them. 172. Leaves are functionally the most active of the vegetable organs. In them the process of digestion or assimilation is chiefly conducted. 173. Assimilation is the name given to the process which accom- plishes these following results :— 1. The chemical decomposition of the oxygenated matter of tho sap, and of the carbonic acid which is absorbed by the leaves; re- sulting mainly in the liberation of pure oxygen, the fixation of carbon and the elements of water in the tissues of tho plant. Thus, through the powers of life, and under the influence of solar light and heat, oxygen, hydrogen, nitrogen, and carbon, or some of these, uniting in certain proportions, become gluten, chlorophyll, gum, sugar, or starch, etc.; and in like manner all other vegetable compounds pass from dead matter into the condition of living substance. 174. The oxygen liberated by plants during the process of assimilation passes into the air ; and as assimilation is constantly going on during sim- light, the amo\mt of oxygen thus poured into the air by plants is enormous, and indeed, so far as we know, vegetation is the only great operation in natui’e which restores to the air that free oxygen gas which is consumed by animals, and in all processes of combustion is indispensable to animal Ufe. III. Classification. 175. It has been already said (2) that descriptions of plants should be arranged, as nearly as possible, under natural divisions, so as to facilitate the comparison of each plant with those most nearly allied to it. Tho descriptions here alluded to are descriptions of species ; the natural divisions of the Flora refer to natural groups of species. 176. A species comprises all the individual plants which resemble each other sufliciently to make us conclude that they may all have deseendod from a common parent stock. These individuals may often differ from each other in striking particulars, such as colour of flower, size of leaf, etc.; but such difliercnces, observation teaches us, may occur in seedlings, raised from one individual. 177. When a number of individuals of a species differ from the majority in any striking particular, they constitute a vari^y. If the variety generally comes true from seed, it is often called a race. 178. A Variety can be propagated with certainty only by grafts, cut- tings, bulbs, or tubers, or other method which produces a new plant by the growth of buds taken from the old one. A race may very frequently, but not with certainty, be proi)agatod by seed. 179. Tho known species of plants (now nearly 100,000) are far too numerous to bo studied %vithout classification. To facilitate their study, an admirable system, invented by Linmeus, has been universally ado])t(!d, viz. one connnon, substantive name is given to a number of species which