26 Plant Physiology 



of relatively thin-walled cells which may have undergone 

 very little, though sometimes considerable, change in 

 shape. They are generally more or less rectangular or 

 polygonal in outline, and frequently exhibit large inter- 

 cellular spaces. In some cases, especially when the proto- 

 plasm has been lost, the walls may be infiltrated with 

 mineral matters. In situations where they may be 

 directly or indirectly exposed to the drying action of the 

 air, the walls may contain cutin or suberin, thus rendering 

 them less penetrable to the passage of water. If the 

 walls are thickened at the angles, as in the supporting 

 cells of the cortex, they are commonly termed collen- 

 chyma. 



Parenchyma of some form almost invariably accom- 

 panies conductive tissues, but it is not particularly adapted 

 for the rapid movement of solutions, being in large part 

 dependent upon simple diffusion phenomena. It has been 

 found, however, that in the parenchyma there are com- 

 monly minute cytoplasmic connections between adjacent 

 protoplasts; that is to say, minute pores may occur in 

 walls separating cells, and through these pores cyto- 

 plasmic fibrils may extend, connecting therefore adjacent 

 protoplasts. These connections may be of great impor- 

 tance in the relations existing between the cells in paren- 

 chymatous tissue. 



19. Sclerenchyma. -This term is usually employed to 

 denote cells with considerably thickened walls. Thick- 

 ening may proceed to such extent that the protoplast dis- 

 appears and the lumen may be practically closed. The 

 grit cells of the fruit of pear are much thickened, and the 

 stone cells of the " pits " of drupaceous fruits, or of the 



