58 PARENCHYMA CELLS 



of twelve- or fourteen-sided polyhedra. But, not all cells desig- 

 nated as "parenchyma" are isodiametric in shape. As exam- 

 ples, the following may be cited: the narrowly "cylindrical" 

 form of palisade parenchyma cells, the lobed form of "arm-pali- 

 sade" cells (cf. Meyer, 1923, p. 16) and the elongated shape of 

 the cells in vascular rays. 



2. Structure and chemistry of the wall. During the differ- 

 entiation of the parenchyma cells in the cortex and pith of stems 

 and in the mesophyll of leaves, little or no appreciable increase in 

 Avail thickness occurs and a true secondary wall, clearly defined 

 from the original primary wall, may be absent. In such cells, 

 the thin primary wall seems to consist largely of cellulose. Sim- 

 ple pits are present but they are often restricted to certain local 

 regions of the wall (cf. De Bary, p. 117, Fig. 46). In contrast, 

 Avood-parenchyma cells and the cells of xylem rays are often pro- 

 vided with relatively thick walls, which are abundantly pitted. 

 According to Eames and MacDaniels (p. 68), the parenchyma 

 cells of secondary wood often have "thick, more or less strongly 

 lignified walls." Whether the thick areas of the walls of these 

 cells are "secondary" or "primary" in nature apparently con- 

 stitutes an open problem at present. 



3. The protoplast . The retention at maturity of an active 

 protoplast represents one of the most important characteristics 

 of parenchyma cells. Indeed, because of this fact, parenchyma 

 cells perform many of the most fundamental physiological proc- 

 esses, notably photosynthesis, food and water storage and secre- 

 tion (Meyer, 1923; Netolitzky. 1935; Sperlich, 1939). In addi- 

 tion to their important metabolic activity, however, parenchyma 

 cells possess to an exceptional degree the ability to revert to a 

 meristematic state (Ilayward. 1938, p. 14) . This is clearly shown 

 by the i-apid response of parenchyma tissue to tlic^ ])liysical and 

 chemical effects of artificial or natural wounding. The nature of 

 such responses is highly variable and complex, ranging from the 

 production of "callus" or of cork tissue, to the regressive forma- 

 tion of root or bud primordia [cf. Priestley and Swingle (1929) 

 and Bloch (1941)]. It may be argued that the ease with which 

 parenchyma cells can be induced to divide and to produce new 

 tissues and organ primordia is evidence of their "primitive" or 



