Chapter VI 



THE PLASTIDS 



The plastids (Fr. plastes, plastides, leucites) :• It has been known 

 for a long time that chlorophyll is found localized in small bodies 

 which were first observed by Meyen (1828) and described by him 

 under the name of chlorophyll grains. These bodies were further 

 studied by numerous workers on the cells of the phanerogams, 

 where they appear scattered in the cytoplasm as numerous globular 

 or lenticular bodies measuring from 6-lOft in diameter. Nageli 

 (1846) first noticed that they multiplied by division. Later, VON 



MOHL (1838-1856), and then Sachs (1852), es- 

 tablished the fact that these bodies are composed 

 of a substratum which is colorless, insoluble in 

 alcohol, protein in nature and which, like the cyto- 

 plasm, stains yellow with an aqueous solution of 

 iodine. On this substratum is fixed the chloro- 

 phyll which can be dissolved in alcohol, leaving be- 

 hind the colorless substratum. voN Mohl and 

 Sachs finally proved that these bodies are the 

 seat of starch formation. In experiments, now 

 classical, Sachs succeeded in showing that the 

 starch grains which are formed in the chlorophyll 

 grains are the direct products of assimilation in 

 the presence of chlorophyll. These bodies were 

 later the object of splendid research in the 

 phanerogams by Schimper (1883) and Meyer 

 (1883), who will be spoken of later. These bodies 

 were called chloroplastids (Schimper), autoplasts 

 (Meyer), chloroleucites (van Tieghem) or chlo- 

 roplasts (Errera). It is not necessary to stress 

 the importance of these bodies which are the cen- 

 ter of the formation of chlorophyll, starch grains 

 and many other products and which play an essen- 

 tial role in photosynthesis. 



The evolution of these chlorophyll bodies in 

 the higher plants, the pteridophytes and phanerogams, remained 

 obscure for a long time. It is known, indeed, that in these plants 

 chlorophyll is not generally found in the egg or in embryonic cells 

 and appears only in the course of cellular differentiation and then 

 only in tissues exposed to the light. In embryonic tissue and in 

 all root tissue, chloroplasts are generally not encountered. The 

 question as to the origin of these bodies was raised but it was not 

 possible to answer it until very much later. 



In the algae, chlorophyll is always contained in all parts of 

 the thallus and, in consequence, chloroplasts are found in all the 



Fig. 10. — Euglena 

 viridis. St, stigma 

 (eyespot). Fl, flagel- 

 lum. Ch, chloro- 



plasts. P, paramy- 

 lum. 



