stamens 



INTKODUCTION. XXxI 



218. Leaves are fimetionally the most active of the orgnns of vegetation. In them 

 IS chiefly conducted digestion or Jssimllation, a name given to the process which 

 accomplishes the follomng results : — 1. The chemical decomposition of the oxygenated 

 matter of the sap, the absorption of carbonic acid, and the liberation of pure oxygen 

 nt the ordinary temperature of the air. 2. A countei'-opemtion 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 phint. The exlialation of oxygen appears to take place imder the hiflu- 

 ence of solar heat and liglit, chiefly fi-om the under surface of the leaf, and to ho In 

 some measure regulated by the stomates; the absorj>tion of oxygen goes on always in 

 tiie dark, and hi the daytime also in certain cases. The transformation of tlie sap is 

 enected within the tissues of the leaf, and continues pixtbably more or less tliroughout 

 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 eomlla) acts in the first instance in protecting tlie 

 ns anrl pistils during the early stages of their development. When expanded, 



the use of the brilliant coloui-s which they often display, of the sweet or strong odours 

 they emit, lias not been adequately explained. Perhaps they may have great influence 

 in attracting those insects whose concurrence has been shown in many cases to be ne- 

 cessary for tlie due transmission of tlie pollen froin 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 tlie organs themselves, or witli the 

 assistance of wind, of insects, or other extraneous agents, liave been the subject of 

 numerous observations and experiments of the most distinguished naturalists, and are 



et far from being fully investigated. Their details, however, as far as known, would 

 •e far too long for the present outline. 



222. Tlie fruit nourishes and protects the seed until its maturity, and then often 

 promotes its dispersion by a great variety of contrivances or apparently collateral cir- 

 cumstiinces, e.^. by an elastic dehiscence which casts the seed off to a distance ; by 

 the development of a pappus, wings, hooked or other appendages, wliich allows them 

 *o Lc carried off by winds, or by animals, etc., to which tliey may adiiere ; by their 

 small specific gravity, which enables them to float down streams ; by their attractions 

 to birds, etc., who taking tliem for food drop them oRen at great distances, etc. Ap- 

 pen^ngf^ to the seeds themselves also often promote dispersion. 



^ 223. Hairs have various functions. The ordinary indumentum (171) of stems and 

 leaves indeed,seenis to take little part in the economy of the plant besides peihapa 

 ^ome occasional protection against injurious atmospheric influences, but the root-hairs 

 (216) are active absorbents, the hairs on stvles and other parts of flowers appear often 

 i^iatcrially to assist the transmission of pollen, and the exudations of glandular hairs 

 U^o, 2) are often too copious not to exercise some influence on the plicnomena ot 

 vegetation. The whole question, however, of vegetable exudations and their influence 

 o" tlie economy of vegetable life, is as yet but hnperfectly understood. 



Chap. IY. Collection, Pkeskkvatioit, and DEXi-E^riyATioN or Plants. 



224. Plants can undoubtedly be most easilv and satisfactorily exammed wlien freshly 

 pHiered. But time wiU rarely admit of this being done, and it is moreorer desiraWe 

 Jo <^ompare them with other plants pi-eviously observed or collected. Specimens must, 

 Therefore, be selected for leisurely observation at home, and preserved for future re.er- 

 ^nce, A collection of such specimens constitutes a Herharitm. 



225, A botanical Specimen, to bo perfect, should have root, stem, leaves Jlowers 

 (both open and in the bud), and fruit (both young and mature) It is not, however 

 ^^^-^''J's possible to gather such comj.leto specimen^ but tJic caUector should aim at 



