THE CELL 



gases but not to fluids. Below the cuticle lies the cell wall 

 proper, in this case composed of cellulose. The cuticle and 

 its underlying wall are continuous except in the region of certain 

 apertures, the stomata, one of which is represented in the figure. 

 From the stoma there passes inward an air space, which quickly 

 divides into fine canals lying in the angles between the internal 

 cells. These are intercellular spaces and are a practically unfailing 

 accompaniment of the living elements in vascular plants. 



The next illustration (Fig. 2) 

 visualizes the conditions in a harder 

 tissue, namely, one of the wood rays 

 of a dicotyledon. Here the mass of 

 cells present individually somewhat 

 rounded contours, and in the result- 

 ing angular interstices appear the 

 intercellular spaces. The protoplasm 

 in the cells under consideration is 

 much denser than in the elements of 

 the wall of the ovary figured above, 

 and accordingly the nucleus lies near 

 the geometrical center of the cell, as 

 is commonly the case in highly proto- 

 plasmic elements, in contrast to its 

 'peripheral position in those in which the protoplasm is only 

 a bounding utricle. The cell wall in the present instance is, rela- 

 tively to the size of the cells, much thicker than in the first figure. 

 As a consequence of this increased thickness of the wall, special 

 devices are necessary for the purpose of permitting interchanges 

 between the cellular elements and their environment. The wall 

 is thin in certain definite regions for the attainment of this end, and 

 these locally thin spots are known as pits. It is clear that the pit 

 of one cell always coincides with a corresponding pit in a neighbor- 

 ing element. Further, it is obvious that, although the cells are 

 in close relation to one another by means of their pits, these are 

 prevented from becoming actual holes by the persistence of the 

 middle lamella or cement substance as the membrane of the pits. 

 Not all the pits, however, meet a corresponding depression. It 



FIG. 2. Tangential section of 

 cells of a ray in a dicotyledon 

 (Drimys species), showing pitting 

 in relation to cells and intercellu- 

 lar spaces. 



