GENERAL 87 



to pass throughout the entire plant, although the living protoplasts do 

 not absorb either of these pigments. Hence substances which are ab- 

 sorbed only when they reach the protoplasts of a given tissue, may be 

 largely conveyed thither by the dead elements through which the water 

 current passes, while so long as an active current is maintained, the 

 protoplasts contiguous to the conducting channels may absorb only a 

 fractional amount even of the dissolved substances which are capable of 

 endosmosis. Hence in this way soluble materials may attain a wide 

 distribution without any direct transference from protoplast to protoplast 

 being necessary. The protoplast of a cell in the interior of a tissue draws 

 the substance it requires from the imbibed fluid of the adjacent cell-walls 

 in much the same way as would the protoplast of a cell swimming freely 

 in water. Similar relationships hold good for substances excreted by the 

 cell, independently of whether the excrete products are directly eliminated 

 or are deposited in more or less distantly situated tissues. 



When a continuous epidermal layer covers the surface of the plant, 

 absorbed substances must pass through this layer before penetrating to the 

 interior of the plant, but when openings such as stomata and lenticels are 

 present, substances may penetrate without necessarily undergoing imme- 

 diate absorption. Stomata and lenticels, so widely distributed in the higher 

 plants, are of the utmost importance for gaseous interchange as well as for 

 the excretion of water vapour, and in many plants, locally distributed water 

 stomata permit an excretion of fluid water. Moreover, by means of 

 communicating intercellular spaces, gases may reach the innermost cells 

 and tissues without having been absorbed by any of the more peripherally 

 situated cells or cell-walls. Living or dead elements, in the form of either 

 specially elongated cells, or of vessels produced by the longitudinal union 

 of rows of cells, are largely employed for the rapid transport of water and 

 dissolved substances to distant parts. All such arrangements are of the 

 highest importance, in so far as they render possible a sufficiently rapid 

 transference of the substances required in. metabolism, and it is by these 

 arteries, so to speak, that the cells and tissues in which metabolism is 

 active are supplied with food material, the conducting channels being 

 subordinate to the organs they supply. In the higher plants, as well as 

 in the lower ones, absorption and excretion are not, in the true sense, 

 functions of the entire organism, but of individual cells, and it is with this 

 exchange of substance occurring in the individual cell that we are im- 

 mediately concerned. In attacking this problem, we are at once confronted 

 by all those difficulties which arise when we attempt to solve any problem 

 involving a knowledge of the inherent nature of the vital mechanism. 



In order to reach the cell-sap in the interior of a cell, fluids and 

 dissolved substances must pass by diosmosis first through the cell-wall, 

 when one is present, then through the external plasmatic limiting mem- 



