ELEMENTARY 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 accomplishes 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 ordinary 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 composition of the plant. The exhalation of oxygen appears 
to take place under the influence of solar heat and light, chiefly from the under 
surface of the leaf, and to be in some measure regulated by the stomates ; the ab- 
sorption of oxygen goes on always in the dark, and in the day time 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. Perhaps they may have great 
influence in attracting those insects whose concurrence has been shown in many cases 
to be necessary 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 
conveyed 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 naturalists, 
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 collateral 
circumstances, e.g., 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 great dis- 
tances, etc. Appendages to the seeds themselves also often promote dispersion. 
223. Hairs have various functions. The ordinary indumentum (171) of stems and 
leaves indeed seems to take little part in the economy of the plant besides perhaps 
some occasional protection against injurious atmospheric influences, but 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 hairs (175, 2) are often too copious not to exercise some influence on the 
phenomena of vegetation. The whole question, however, of vegetable exudations and 
their influence on the economy of vegetable life, is as yet but imperfectly understood. 
Chap. IV. Collection, Preservation, and Determination of Plants. 
224. Plants can undoubtedly be most easily and satisfactorily examined when 
I freshly gathered. But time will rarely admit of this being done, and it is moreover 
I desirable to compare them with other plants previously observed or collected. Speci- 
mens must, therefore, be selected for leisurely observation at home, and preserved 
} for future reference. A collection of such specimens constitutes a Herbarium. 
225. A botanical Specimen, to be perfect, should have root, stem, leaves, flowers 
I (both open and in bud), and fruit (both young and mature). It is not, however, 
always possible to gather such complete specimens, but the collector should aim at 
completeness. Fragments, such as leaves without flowers, or flowers without 
! leaves, are of little or no use. 
