Guilliermond - Atkinson 



— 76 — 



Cytoplasm 



starch grains stained by the iodine. Moreover, simultaneous stain- 

 ing of the starch and the chondriocont may be obtained by various 

 more complicated processes. Milovidov, especially, has shown how- 

 to make such permanent preparations. These methods are much 

 more delicate and do not give constant results. 



Starch formation takes place in the same way in the greatest 

 variety of tissues which are without chlorophyll: roots, tubers, 

 epidermis. Yet there are cases, as in the tuber of the potato, in 

 which the chondriome is represented only by mitochondria which 

 elaborate starch after having undergone a slight increase in vol- 

 ume. In such cases they take on the appearance of vesicles, due 

 to the production in their interior of a starch grain which mito- 

 chondrial methods do not stain. Sometimes the chondrioconts 

 which will later elaborate starch may acquire, before its production, 

 a much more marked increase in volume which makes it possible 

 to distinguish them very clearly from the other chondriosomes in 



the mature tissue. This is 

 seen, for example, in the root 

 of Phajus grandifolius in 

 which by following the meri- 

 stem to the region of differ- 

 entiation, it is seen that some 

 of the chondrioconts take the 

 form of rods or spindles. 

 These chondrioconts are very 

 clearly bigger than the chon- 

 driosomes which continue to 

 exist side by side with them 

 but without increasing in size. 

 These enlarged elements cor- 

 respond to the amyloplasts 

 described by Schimper and 

 Meyer, through the agency of which the grains of starch arise. It 

 seems that the increase in volume is due to the formation in the 

 chondriocont of a needle-shaped protein crystal lying along the 

 long axis of the element, whose contours follow that of the crys- 

 tal. In other cases the amyloplasts assume the appearance in 

 mature cells of rather long rounded bodies (hairs of Tradescantia 

 virginiana) . It may be added that the simple or compound starch 

 grains instead of arising in the center of the swelling of the chon- 

 driosome, chondriocont, or mitochondrium, may form on its periph- 

 ery. The chondriosome then bears a vesicle whose wall is much 

 thicker on one side than on the other. The starch grain which 

 occupies the vesicle increases in size and ends by bursting out of 

 the chondriosome which is thus reduced, little by little, to a thin 

 cap, covering the starch grain in the region most distant from its 

 hilum (potato tuber, root of Phajus grandifolius). The starch 

 grain thus formed no longer remains surrounded by a continuous 

 mitochondrial layer as in the preceding case (Figs. 40, 41) . 



• C C 



Fig. 39. — Stages in starch formation in 

 potato tubers. 



