Differentiation 193 



observable in the cell, is usually reflected in increased cell size and may 

 be a factor in other changes which occur during differentiation. How 

 important this factor is in cellular differentiation is not known. It may 

 account for some of the diversity in cell size but probably has little to do 

 with other aspects of differentiation ( p. 441 ) . 



The Cell Wall. Some of the most distinctive ways in which cells differ 

 are concerned with the cell wall. The wall is of much greater variety and 

 significance in plant cells than in animal cells, and the key to cellular 

 differentiation in plants is often to be found in it. Walls may differ greatly 

 in thickness, chemical composition, and structure, depending upon the 

 function of the cells of which they are parts. In certain tissues the cells 

 die early, and only the thickened walls which they formed remain. The 

 size and shape of the cell and the manner of its growth seem often to 

 be dependent primarily upon the character of the wall. Studies of the 

 chemistry and fine structure of the wall show how complex its consti- 

 tution may be and make clear that any detailed analysis of cellular dif- 

 ferentiation must pay attention to changes not only in the living material 

 of the cell— the true protoplast— but in the wall that is the result of its 

 activity (Bailey and Kerr, 1935; Frey-Wyssling, 1955). 



In a few cases the origin of differentiation in the wall may be observed, 

 especially where sculpturing occurs, as in the ringed, spiral, and retic- 

 ulate cells of the xylem and in other tissues with similarly unequal 

 wall thickening. Criiger (1855) and Dippel (1867) many years ago 

 showed that the first indication of where such thickenings were to occur 

 in developing cells was the accumulation of cytoplasm, more densely 

 granular than the rest, in a definite pattern. The thickenings of the 

 wall (rings, reticulations, or others) were laid down in close relation to 

 this cytoplasmic pattern (Barkley, 1927). Strasburger (1882) observed 

 streaming of cytoplasm along these strands. Large and vacuolate paren- 

 chyma cells that are being redifferentiated as reticulate xylem cells in re- 

 generation are particularly good material in which to observe the 

 cytoplasmic network upon which the wall reticulum is being built ( Sin- 

 nott and Bloch, 1945; Fig. 8-5). Kiister ( 1931) called attention to the simi- 

 larity between such cytoplasmic configurations and Liesegang rings. 

 Denham (p. 166) reports that in many cells the directions of cytoplasmic 

 streaming has a definite relation to the micellar configuration of the wall. 



Differentiation and Position. In all the cases of cellular differentiation 

 here described, the position of the cell in the developing system is evi- 

 dently closely concerned with the type of differentiation that it under- 

 goes. A notable example is the formation of reaction wood, which dif- 

 ferentiates in the precise position where it will tend to bring a terminal 

 or lateral axis into a specific orientation in the pattern of the whole 

 (p. 356). 



