Chapter XI — 117 — Role of Ghondriosomes 



the role of these organelles. Nevertheless, the fact that the chon- 

 driosomes usually show no morphological evidences which can be 

 related to their participation in secretory phenomena does not 

 exclude the possibility that they may have a role in these phenom- 

 ena, and it is possible that later studies may succeed in demon- 

 strating this role which is suggested by the close parentage of 

 chondriosomes and plastids. So all cytologists are at present agreed 

 in attributing to the chondriosomes some important role in cellular 

 metabolism. 



The role of the plastids in chlorophyll-bearing plants is, on the 

 contrary, very clear, for it is manifested morphologically by the 

 production within these organelles of chlorophyll, carotinoid pig- 

 ments, starch grains and so forth. However, we do not know at 

 all by means of what physico-chemical processes these phenomena 

 are brought about. At first an essential role in the elaboration of 

 these different products, as well as participation in the phenomenon 

 of photosynthesis, was attributed to the plastids. The plastids 

 were considered as small laboratories which were the seat of the 

 most important synthesis in the plant cell. 



At the present time there is a tendency, rightly or wrongly, to 

 react against this way of thinking and to consider the plastids as 

 perhaps only the accumulation centers for certain substances manu- 

 factured by the cytoplasm itself. In any case, the work of modern 

 physiologists caused it to be thought that the plastids contribute 

 only in part to photosynthetic phenomena. Actually, we still do 

 not know how much to attribute to the chlorophyll, how much to 

 the substratum of the plastids and how much to the cytoplasm. 

 Nevertheless, it seems that the plastids do indeed play an important 

 part in photosynthesis. In any case, there is a role which cannot 

 be denied the plastids. It is the ability to condense the hexoses 

 into starch. In this case it is a question not of accumulation but 

 of actual synthesis. The influence of plastids on the form and 

 growth of starch grains is manifested by the fact that these prop- 

 erties are determined by the place where the grain appears in the 

 plastid. If the grain arises in the middle of the plastid, the hilum, 

 i.e., its oldest part, is the center and concentric layers, which are 

 the outcome of growth by apposition, develop regularly about it. 

 The starch grain in this case remains entirely enveloped by a thin 

 mitochondrial layer. When, on the contrary, the growing grain 

 forms on the periphery of the plastid, it very soon bursts out of the 

 plastid which then covers it as a sort of cap at only one extremity. 

 In this case the hilum is situated at the pole opposite to that occu- 

 pied by the cap, and the layers of growth do not form any longer, 

 except in those regions which are still in immediate contact with 

 the plastid. The grain then is eccentric in structure. 



The fact that the starch grain is formed and is hydrolyzed in 

 the interior of the plastid led certain investigators to believe in the 

 existence in the substratum of the plastid of a diastase of rever- 

 sible action, capable of bringing about both synthesis and hydro- 

 lysis of starch (Meyer, Salter). Maige, on the contrary, thinks 



