OUTLINES OF BOTANY. 
XXXI 
218. Leaves are functionally the most active of the organs of vegetation. In them 
is chiefly conducted digestion or Assimilation , a name given to the process which ac- 
complishes the following results : — 1. The chemical decomposition of the oxygenated 
matter of the sap, the absorption of carbonic acid, and the liberation of pure oxygen 
at the oi’dinary temperature of the air. 2. A counter-operation by which oxygen is 
absorbed from the atmosphere and carbonic acid is exhaled. 3. The transformation 
of the residue of the crude sap into the organized substances which enter into the com- 
position of the plant. The exhalation of oxygen appears to take place under the influ- 
ence of solar heat and light, chiefly from the under surface of the leaf, and to be in 
some measure regulated by the stomates ; the absorption of oxygen goes on always in 
the dark, and in the daytime also in certain cases. The transformation of the sap is 
effected within the tissues of the leaf, and continues probably more or less throughout 
the active parts of the whole plant. 
219. The Floral Organs seldom contribute to the growth of the plant on which they 
are produced ; their functions are wholly concentrated on the formation of the seed 
with the germ of a future plant. 
220. The Perianth (calyx and corolla) acts in the first instance in protecting the 
stamens and pistils during the early stages of their development. When expanded, 
the use of the brilliant colours which they often display, of the sweet or strong odours 
they emit, has not been adequately explained. Perlxaps they may have great influence 
in attracting those insects whose concurrence has been shown in many cases to be ne- 
cessai'y for the due transmission of the pollen from the anther to the stigma. 
221. The pistil, when stimulated by the action of the pollen, forms and nourishes 
the young seed. The varied and complicated contrivances by which the pollen is con- 
veyed to the stigma, whether by elastic action of the organs themselves, or with the 
assistance of wind, of insects, or other extraneous agents, have been the subject of 
numerous observations and experiments of the most distinguished natui’alists, and are 
yet far from being fully investigated. Their details, however, as far as known, would 
be far too long for the present outline. 
222. The fruit nourishes and protects the seed until its maturity, and then often 
promotes its dispersion by a great variety of contrivances or apparently collatei’al cir- 
cumstances, e.ff. by an elastic dehiscence which casts the seed off to a distance; by 
the development of a pappus, wings, hooked or other appendages, which allows them 
to be carried off by winds, or by animals, etc., to which they may adhere ; by their 
small specific gravity, which enables them to float down streams ; by their attractions 
to birds, etc., who taking them for food drop them often at gi’eat distances, etc. Ap- 
pendages to the seeds themselves also often promote dispersion. 
223. Hairs have vai'ious functions. The ordinaxy indumentum (171) of stems and 
leaves indeed seems to take little part in the economy of the plant besides perhaps 
some occasional protection against injxxrious atmospheric influences, buit the root-hairs 
(216) are active absorbents, the hairs on styles and other parts of flowers appear often 
materially to assist the transmission of pollen, and the exudations of glandular hail’s 
(175, 2) are often too copious not to exercise some influence on the phenomena of 
vegetation. The whole qxxestion, however, of vegetable exudations and their influence 
on the economy of vegetable life, is as yet but imperfectly undei'stood. 
Chap. IY. Collection, Pbesebvation, and Deteemination oe Plants. 
224. Plants can undoubtedly be most easily and satisfactorily examined when freshly 
gathered. But time will rarely admit of this being done, and it is moreover desirable 
to compare them with other plants previously observed or collected. Specimens must, 
therefore, be selected for leisui-ely observation at home, and preserved for futxu-e refer- 
ence. A collection of such specimens constitutes a Herbarium. 
225. A botanical Specimen, to be perfect, should have root , stem, leaves, flowers 
(both open and in the bud), and fruit (both young and mature). It is not, however, 
always possible to gather such complete specimens, but the collector shoxxld aim at 
