Guilliermond - Atkinson 



— 96 — 



Cytoplasm 



But these are rare and almost diagrammatic examples. In most 

 cases, it is absolutely impossible among the elements which consti- 

 tute the chondriome in cells of the meristem to distinguish those 

 which will become plastids from those which will remain inactive, 



>5 n> •' 



I f '^ v.. A 



B 

 Fig. 62 (left). — Chondriome, showing morphological similarity of 

 the meristem of pea root (A) and of pancreas cells of gruinea pig 

 (B), each fixed by Regaud's method and stained with iron haema- 

 toxylin. A, plastids and chondriosomes indistinguishable; B, chon- 

 driosomes and Claude Bernard granules. (After Cowdry). 



Fig. 63 (right). — Detail of chondriosomes (A) in pea root and 

 (B) in mouse pancreas. X 1687. (After Cowdry). 



for they are of similar form. Although in most cases it is the 

 chondrioconts which become plastids, there are numerous excep- 

 tions, and cases are found in which the granules, as well as the 

 chondrioconts, become plastids. It may even happen in some cases 

 that the granules alone form the plastids, whether the other ele- 

 ments present have the form of chondrioconts or whether they 

 also have the form of mitochondria. In the tuber of potato, for 

 example, only granular mitochondria are found (Fig. 39) and it is 

 through the agency of some of these that starch is elaborated. 

 Furthermore, the distinction which we have made between the two 

 categories of elements, i.e., chondrioconts and granules, in the bud 

 of Elodea canadensis and in the root of Cucurbita Pepo, is far from 

 being general. Whether we can differentiate between the two cate- 

 gories depends, in Elodea, upon the state of activity of the bud and 

 the period at which it was collected. There are buds in which this 

 distinction is much less clear, others where it can no longer be 

 made. The difference in size which we have noticed between the 

 starch-forming plastids and the chondriosomes in parenchyma cells 

 of the pumpkin root may itself diminish or disappear. For ex- 

 ample, in cells in which the starch grains have just been digested, 

 the chondrioconts which elaborated them, grow thinner and are in- 

 distinguishable from the other elements of the chondriome. Cells 

 in the meristem of Elodea and of the pumpkin are derived from 

 other embryonic cells in which the two categories are not dis- 

 tinguishable. 



