INTRODUCTION. XXxiii 



chiefly of carbonic acid and nitrogen or nitrogenous compounds dissolved in water, 

 3. In some cases roots secrete or exude small quantities of matter in a manner and with 

 a purpose not satisfactorily ascertained. 



217. The Stem and its branches support the leaves, flowers, and fruit, transmit the 

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

 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 tissues, 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 cliiefly 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 ordinary temperature of the air. 2. A counter-operation by which oxygen is ab- 

 sorbed 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 compo- 

 sition of the plant. The exhalation of oxygen appears to take place under the influence 

 of solar heat and light, cliiefly 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 tlie 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 briUiant 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 neces- 

 sary for the due transmission of tlie 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 compUcated contrivances by which tlie 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 naturahsts, 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 matu^rity, and then often 

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

 cumstances, 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 

 smaU 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. Appen- 

 dages 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. 



