Differentiation 



197 



The exact localization of the endodermis makes it of particular interest 

 morphogenetically. Light is important in its development, for it is well 

 differentiated in roots and etiolated stems and much more poorly de- 

 veloped in the light (Bond, 1935). Venning (1953), however, finds that 

 factors other than light are responsible for the formation of a typical 

 endodermis. Van Fleet in a series of papers has studied differentiation 

 histochemically ( 1954a and b ) , with particular reference to the position 

 of the endodermis on an oxidation-reduction gradient as well as to the 

 distribution of various enzymes. He has stressed the importance of histo- 

 chemical determination of enzyme distribution (1952) as a means of dis- 

 covering chemical differentiation before it is evident in structure (Fig. 

 8-9). 



Fig. 8-9. Chemical differentiation of the endodermis. Its cells stain differently from 

 those of adjacent tissues. ( From Van Fleet. ) 



Fiber Patterns. An example of the differentiation of a somewhat more 

 complex pattern but one consisting of a single type of cells is provided 

 by the development of a system of fiber strands such as that found in the 

 pericarp of the cucurbit fruit, and especially well developed in the 

 "dishcloth" gourd, Luff a (Sinnott and Bloch, 1943). Here the pericarp 

 tissue in the early ovary primordium consists of longitudinal rows of 

 squarish parenchyma cells with most of the divisions at right angles to 

 the axis of the young ovary or parallel to it. Here and there begin to 

 occur divisions not in these two orientations but obliquely at various 

 angles (Fig. 8-10). Parallel to each such division is a series of others so 

 that in a given cell or its neighbors several elongate and parallel cells 

 are cut out. This group becomes connected with other groups in a con- 

 tinuous series, though successive members of this series may arise at 

 somewhat different angles. The result is that strands of cells are formed, 

 twisting about through the original rectangular cellular system and 



