Chapter VI 



47 — 



The Plastids 



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All these organelles - leucoplasts, chloroplasts, chromoplasts - 

 belong therefore to the same category of elements to which SCHIM- 

 PER gave the general term plastids which we shall replace here by 

 that of plast (Fr. plastesy. They have multiple potentialities, 

 manifestations of which vary with the organ in which the plastids 

 are found. They may remain in the state of leucoplasts and have 

 as their only function the condensation of hexoses into starch, 

 playing the role of amyloplasts; or else they may be transformed 

 into chloroplasts or chromoplasts; or from chloroplasts they may 

 be changed into chromoplasts. They all have 

 the ability to elaborate starch and to produce 

 protein crystalloids within themselves in the 

 manner already described for leucoplasts of 

 the root of Phajus grandifolius and for the 

 chromoplasts of the fruit of Lonicera Xylos- 

 teum. Thus chloroplasts of Cerinthe minor 

 are traversed by a needle-shaped crystalloid 

 of protein which is prolonged freely at its 

 two extremities. 



SCHIMPER considers that starch can not 

 arise directly in the cytoplasm but is always 

 the product of plastidial activity. He de- 

 scribes in all details the process of starch 

 formation through the agency of the plas- 

 tids whether they be leucoplasts or chloro- 

 plasts. Let us recall that starch grains grow 

 by apposition and are made up of a dark 

 hilum surrounded by alternately light and 

 dark layers. The hilum is the portion first 

 formed and the alternate layers correspond 

 to zones of different water content which are 

 developed about the hilum during the growth 

 of the starch grains. The hilum may occupy 

 the center of the grain (central hilum) and 

 is then surrounded by regular concentric lay- 

 ers, or it may be situated at one of the poles 

 of the grain whose concentric layers widen 

 and become more and more numerous at the 

 opposite pole (eccentric hilum). There are simple starch grains, 

 compound starch grains, i.e., several grains stuck together, and 

 half-compound starch grains which are separate at first but be- 

 come united by common concentric layers. The difference in type 

 depends upon the method of formation within the plastid. The 

 simple grain with central hilum arises in the middle of the plastid 

 as a small granule which grows, forming regular concentric layers 

 about the hilum, and remains surrounded on all sides by a plastidial 

 wall which grows thinner as the grain enlarges. The simple grain 



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Fig. 16. — Transforma- 

 tions of chloroplasts in meso- 

 phyll cells of petals. A, 

 Lilium tigrinum. 1, caro- 

 tin granules at the periphery 

 of green plastids; 2, starch 

 grains in plastids whose 

 chlorophyll has disappeared; 

 3, completely formed chromo- 

 plasts whose starch has been 

 absorbed. B, Gladiolus var. 

 1-3, chloroplasts with large 

 starch grains; 4-7, xantho- 

 phyll replaces chlorophyll, 

 starch is absorbed, carotin 

 granules appear; 8-13, caro- 

 tin crystals appear. 



^Translator's note. The French words plastides and plastes are rendered in English by the 

 one word plastid, so that this distinction loses its force in translation. "Plast" is used only 

 in compound words such as "chloroplast", "chromoplast", "leucoplast" etc. 



