102 COMPENDIUM OF GENERAL BOTANY. 



behavior of the living jDrimordial utricle in the movement of food- 

 substances from cell to cell is not well understood. It is, however, 

 very clear that a living cell in contact with water will increase its 

 hydrostatic pressure by taking in more water. 



The so-called transitory starch which occurs in the form of 

 small granules within the cells in which starch circulates is of 

 special importance in the circulation of soluble starch. We can 

 somewhat understand the function of this transitory starch M'hen we 

 learn that the precipitation of starch-snbstance in the form of solid 

 granules produces a decrease in the degree of concentration in the 

 surrounding starch solution. As a result new currents are set in 

 motion toward the graimles ; but why these granules are formed at 

 the suitable moment, and why they are again dissolved in order 

 that the current may continue in the same direction and thus make 

 way for new incoming currents and new precipitates of starch-sub- 

 stance, is but little understood. The processes of hydro-diffusion 

 and diosmosis are, of course, frequently combined in the circulation 

 of food-substances. According to Sachs', we may, in general, 

 express ourselves as follows : Every growing part of a plant acts as 

 a centre of attraction for tlie available food-substances; every 

 storage-tissue or receptacle and every assimilating organ acts as a 

 centre of repulsion as compared with the growing portion. 



Many of the earlier and also of the more recent physiological 

 and anatomical investigations, especially tliose of^ Sachs, also those 

 of Haberlandt and of Schimper, throw light upon the conveyance 

 of assimilates from the leaves. We are here concerned jjrinciijally 

 with the adaptive arrangement of the assimilating cells and other 

 leaf-tissues with regard to the vascular systerrt and the mode of con- 

 duction within the tissues named (see Physiological Anatomy of the 

 Assimilating System, Function VI). In the early sixties Sachs had 

 already observed that germinating plants free from starch (etiolated) 

 when brought into the sunlight would soon contain small starch- 

 grains, first in the chlorophyll-grains of the leaf, then also in the 

 conducting tissues of the petioles and internodes; these starch- 

 granules would disappear when the plant was placed in the dark 

 and again reappear when brought into the light. i.\s a result of 

 these investigations by Sachs, and also those of more recent authors. 



' VorlesuDgen, page 439. 



