704 CHEMICAL PROCESSES IN THE PLANT. 



Growth is only possible as a result of assimilation; but the two processes 

 do not usually concur either in time or locality. The assimilated substances may 

 remain in the plant for a longer or shorter time without becoming employed in 

 the growth of cell-walls or in the production of protoplasmic substances 

 (protoplasm or chlorophyll-granules) ; and in this case they are termed Reserve- 

 materials. Every cell, tissue, or organ in which assimilated substances are stored 

 up for subsequent use is called a Reservoir of Reserve-material. The assimilating 

 cell may itself serve as a reservoir for reserve-material (as unicellular Algae or the 

 leaves of evergreen plants) ; but usually a physiological division of labour is 

 effected in the plant of such a nature as to transfer the products of assimi- 

 lation from the organs that contain chlorophyll to other organs or masses of 

 tissue which serve as reservoirs of the reserve-material and give it up to the 

 parts destined for the formation of new organs (buds, the rudiments of roots, or 

 cambium). In Mosses, Vascular Cryptogams, and woody Phanerogams, the tissue 

 of the stem is usually also the reservoir for this purpose ; in perennial herbs and 

 shrubs it is more often the persistent bulbs, tubers, and rhizomes that perform 

 this function. The spores of Cryptogams which have the power of germination 

 always contain a small quantity of reserve-material, at the expense of which the 

 first processes of germination take place ; in Rhizocarpeae and Ligulatae the whole 

 of the prothallium and embryo is produced in this manner. The seeds of Phanero- 

 gams remove much greater quantities of reserve-material from the mother-plant, 

 which are accumulated either in the endosperm or in the cotyledons ; the greater 

 the quantity of this reserve-material the more numerous and the larger are the stems, 

 roots, and leaves which the seedling can produce before it begins to assimilate. 

 The minute seedlings, for instance, of Nicotiana and Campanula may be contrasted 

 with the strong ones of the Bean, Almond, Oak, &c. Since no assimilation takes 

 place in the dark, it is only necessary to allow seeds, tubers, bulbs, rhizomes, &c. 

 to germinate and develope in the dark in order to form an idea of the number 

 and size of the organs which can be formed from the reserve-material. 



Since the organs of assimilation which contain chlorophyll are usually at a 

 distance from the reservoirs of reserve-material and from the growing buds and 

 roots, the products of assimilation have to be conveyed to the localities where 

 they are required and where they are temporarily deposited. Growth and the 

 deposition of reserve-material are therefore necessarily associated with corre- 

 sponding movements of the products of assimilation and of those undergoing 

 metastasis. 



All these statements may be proved without any more accurate knowledge of 

 the substances themselves which are produced by assimilation in the cells that 

 contain chlorophyll and which undergo metastasis. But before entering on this 

 question, we may first of all discuss the other : — whether all the products of me- 

 tastasis are immediately applicable to the building up of new organs ; and if not, 

 what substances furnish the material for the production of cell-walls, protoplasm, 

 and chlorophyll -granules. Among the extraordinarily large number of the pro- 

 ducts of metastasis which are proved by chemical analysis to exist in various plants, 

 there is a comparatively small number of substances the behaviour of which in 

 the growth of the organs and whose universal distribution through the vegetable 



