Chapter VI — 49 — The Plastids 



and that these granules present the histochemical characteris- 

 tics of lipides and not those of aldehydes (Cf. p. 209). 



Just what these granules signify is still very obscure. In 

 cells in which the plastids never form starch, the granules are 

 often considered as assimilation products replacing starch. It has 

 been noticed, sometimes, that these granules appear in large 

 numbers during the period preceding the formation of starch and 

 of chlorophyll and carotinoid pigments, only to disappear as soon 

 as these products have been formed. It was therefore thought 

 that they might be an intermediate product contributing to the 

 formation of starch or pigment. It has also been shown that very 

 frequently similar granules appear in great numbers in plas- 

 tids as they degenerate in the cells of flowers which are beginning 

 to take form. In this case, the presence of the granules can 

 only be explained as a breaking down of the plas- 

 tidial lipoprotein complex, i.e., as a process called 

 lipophanerosis (demasking of lipides. Cf. p. 203, 

 205). 



Finally, large globules have recently been de- 

 scribed in the plastids of various Cactaceae 

 (Cephalocereus, Echinocereus, etc.). They pre- 

 sent the histochemical characteristics of phyto- 

 sterol and form in the plastid exactly as do 

 starch grains. These globules always precede 

 starch formation and disappear the moment that 

 starch appears. It was therefore supposed by ^J^^;i[^n7 ^Sfof 

 those who did the work that these globules of ™^ Veig'^a^ith lip- 

 phytosterol constituted a material which served '^^ granules in the 



. "^ , „, 1 , r^ ■»«•• chloroplasts. 



m the buildmg up of starch (Savelli, Miss 

 Manuel) (Fig. 19). 



For a very long time it was impossible to obtain paraffin sec- 

 tions of preserved and stained plastids, at least of leucoplasts, 

 for they were destroyed by all fixatives then used, and only the 

 much more resistant chloroplasts were obtained. Schimper's and 

 Meyer's observations were therefore made exclusively from very 

 well executed studies of living material. These observations were 

 necessarily incomplete for it is absolutely impossible to distinguish 

 plastids in living embryonic cells and until the present writing they 

 have been seen only with the very greatest difficulty in colorless 

 differentiated tissue, such as the root, and then only in favorable 

 cases. The classical observations of Schimper on the method of 

 starch formation were based for the most part on examples par- 

 ticularly favorable for study, such as the root of Phajus grandi- 

 folius, in which the leucoplasts are very massive. Therefore the 

 theory of SCHIMPER and Meyer was very largely hypothetical and 

 was based chiefly on what seemed to them likely to be true, and 

 on what can be observed in many algae in which the chloroplasts 

 behave like component parts of the cell. 



Numerous workers contested this theory of starch formation, 

 among others, Belzung, who did not succeed in distinguishing 



