ZSS ASSIMILATION. 



plastids of one sort can for a time undertake the office of the 

 plastids of another sort. It explains, partial!}' at least, the in- 

 trusion of chlorophjll grains into parts of the plant where thej- 

 do not seem to propei'ly belong, and accounts for some of the 

 apparent changes which the}- ma}' subsequently undergo. 



768. According to the early investigations of the subject, the 

 chlorophyll granules were regarded as differentiations, at an 

 early stage in the embryo and seedling, from a mass of homo- 

 geneous protoplasm : according to the present ^iew the}' are 

 derivati\es by division from pre-existing plastids.^ When devel- 

 oped in darkness, they are pale yellowish, or even devoid of color. 

 Plants grown iu the dark (compare 788) become green upon 

 ex()osure to the light, ijrovided they are not at the same time 

 kept too cold. The minimum temperature at which they turn 

 green is different for different plants, but may be said to be 

 in general not far from 6° to 10° C. 



Certain Gymnosperms, notably seedlings of Abies and Finns, 

 develop a bright green color in the deepest darkness, provided, 

 as before stated, the temperature is not below a certain point. 



769. Occurrence of the chlorophyll granules. The granules are 

 found only very sparingly in epidermis, being chiefly confined to 

 the guardian cells of stomata. They occur principally in paren- 

 chyma cells, immediately below the epidermis, and seldom out 

 of reach of the light. But they occur also in a few deep-seated 

 structures, for instance, in the thick cortex of some ligneous 

 plants, and in the tissues of not a few embryos. 



770. That chlorophyll granules are found in the interior of 

 some of the lower animals appears reasonably certain, but the 

 green matter does not always present the same characters. Ac- 

 cording to recent authorities, it assumes in most cases, for in- 

 stance in Spongilla and Hydra, the form of minute granules. The 

 pigment agrees in some of its essential properties with that of 

 ordinary chloropliyll.^ In some cases it must still be considered 

 an open question whether the granules may not be (or at least 

 represent) independent organisms dwelling in certain cavities of 



1 The views of Gris (Ann. des So. nat. bot., 1857) may be summarized as 

 follows : The granules arise by differentiation of tlie protoplasm in certain 

 yonng cells into two portions ; one of these assumes the form of roundisli 

 or lenticular bodies (tlie proper granules), which under the influonce of liglit 

 become colored green, while tlie other remains as a matrix in which they are 

 embedded. 



^ For an interesting treatment of this subject, consult Geddes : Nature, 1882, 

 and LankeSter, Journal of the Royal Microscopical Society, 1882, p. 241. 



