THE STRUCTURE OF PLANT CELLS 



6i 



cytoplasm. In a typical mature plant cell the cytoplasm is present as a thin 

 layer lining the inner surface of the cell wall. The two boundary layers of 

 the cytoplasm — that in contact with the cell wall and that in contact with 

 vacuole — are called the cytoplasmic membranes (Chap. X). Imbedded in 

 the cytoplasm are numerous well differentiated bodies known as plastids. 

 Plastids are specialized cytoplasmic structures which are usually centers of 

 certain types of physiological activity. They are commonly classified on the 

 basis of their color into three gi'oups : The leticoplasts which are colorless, the 

 chloroplasts which contain the green chlorophyll pigments (also yellow pig- 

 ments), and the chromoplasts which contain red or yellow pigments. Chon- 

 driosomes, minute rod-like or granular bodies, are also found in the cytoplasm. 

 The significance of these structures is not positively known, although a 

 number of different roles have been ascribed to them. 



A B 



Fig. II. Plasmodesms in cell walls of tobacco: (A) sieve tubes and companion cells, 



{B) epidermal cells of leaf. Redrawn from Livingston (1935). 



Although the cell wall appears to imprison each protoplast, and to effec- 

 tively isolate it from the protoplasm of adjoining cells, actually there is prob- 

 ably a continuation of protoplasm from cell to cell. By certain techniques it 

 can be demonstrated that minute pores extend from cell to cell through the 

 cell walls. These pores often contain cytoplasmic strands which connect the 

 cytoplasms of adjacent cells. These strands are termed plasmodesms (Fig. 11). 

 Livingston (1935) has demonstrated the occurrence of plasmodesms in the 

 walls of cells from a number of different tissues of the tobacco plant, and it is 

 generally supposed that they are of widespread if not universal occurrence in 

 plant cell walls. 



The bulk of the interior of mature plant cells is occupied by a single large 

 cavity, the vacuole, which is filled with cell sap. The cell sap is composed of 

 water in which a great variety of substances are dissolved or colloidally 

 dispersed. 



