OUTLINES OF BOTANY. xlv 



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 ex- 

 halation 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 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 for- 

 mation 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 appen- 

 dages, 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. Appendages to the seeds them- 

 selves 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. IY. — Collection, Preservation, and Determination of 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 more- 

 over desirable to compare them with other plants previously observed or col- 

 lected. Specimens must, therefore, be selected for leisurely observation at home, 

 and preserved for future reference. A collection of such specimens constitutes 

 a Herbarium, 



