OUTLINES OF BOTANY. xhx 



transmit the crude sap, or nutriment absorbed by the roots and mixed with 

 previously organised matter, to the leaves, and re-transmit the assimilated 

 or elaborated sap from the leaves to the growing parts of the plant, to be 

 there used up, or to form deposits for future use (204). The transmission 

 of the ascending crude sap appears to take place chiefly through the 

 elongated cells associated with the vascular tissu'es, passing from one cell 

 to another by a process but little understood, but known by the name of 

 endosmose. 



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 decom- 

 position 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 atmo- 

 sphere and carbonic acid is exhaled. 3. The transformation of the residue 

 of the crude sap into the organised substances which enter into the com- 

 position 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 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 pro- 

 tecting 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 trans- 

 mission 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 ex- 

 traneous agents, have been the subject of numerous observations and expe- 

 riments 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 ap- 

 parently 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 distances, etc. Ap- 

 pendages to the seeds themselves also often promote dispersion. 



223. Hairs have various functions. The ordinary indumertum (171) of 

 stems and leaves indeed seems to take little part in the economy of the 



o 



