238 SUMMARY OF CURRENT RESEARCHES RELATING TO 



production of chlorophyll necessary to this result. These latter con- 

 ditions, however, though not indispensable, indirectly favour epinasty 

 by promoting the development of the palisade-cells. 



Starch-generators and Pigment-bodies. *— A. F. W. Schimper 

 applies the general term Plastid to the solid bodies, chlorophyll-bodies, 

 starch-generators, and pigment-bodies, designating them respectively 

 Chloroplastids, Leucoplastids, and Chromoplastids. Chloroplastids are 

 always formed from leucoj)lastids, by increase of size combined with 

 development of the pigment ; chromoplastids always from leucoplastids 

 and chloroplastids. The plastids frequently have an active life, 

 assimilating or producing starch at the expense of already assimilated 

 materials, multiply by division, &c. But others have either very little 

 vital function or none at all, as in the case of the leucoplastids in the 

 epidermis of most plants, and the chromoplastids of flowers and fruits, 

 and of the carrot. These passive plastids have very often a more or 

 less completely crystalline form, and are doubly refractive. The 

 active plastids are always round in the higher plants. 



Many of these passive plastids have in fact been described as 

 crystals, as those of the carrot and of Neottia nidus-avis. In the carrot 

 they are usually rectangular plates or narrow rhombs, to which starch- 

 grains are attached. In form and optical properties they agree with 

 crystals of the rhombic system. The brown chromoplastids of Neottia 

 are also often of regular rhombic or triangular form, and also usually 

 have starch-grains attached to them. In other flowers and fruits are 

 two- or three-pointed plates, the former oval, fusiform, or acicular ; 

 less often the chromoplastids are rod-shaped and rounded at the ends. 

 But the plastids have sometimes less regular crystalline forms, as those 

 of the carrot and Neottia, but still are distinctly crystals, and doubly 

 refractive. In some flowers, like those of Tropceolum and Echeveria, 

 the chromoplastids are much less regular and crystalline, and in 

 Asphodeline have even a rounded outline with indications of two or 

 three edges. In Iris Pseudacorus the plastids are disk-shaped or 

 irregular, or of a rounded triangular form. In other flowers and 

 fruits the plastids are quite round. 



The leucoplastids are sometimes also doubly refractive, and vary 

 in form between flatly fusiform and rod-shaped. Kegularly fusiform 

 chloroplastids occur in the epidermis of leaves, where, at an early 

 stage, they are colourless. These crystalline leucoplastids agree in 

 everything except colour with chromoplastids, and sometimes become 

 directly transformed into them. 



Although these plastids would be determined by the crystallograph 

 to be crystals, yet they differ from true crystals in some important 

 points. They are composed, at least sometimes, of vital protoplasm, 

 although it may sometimes be dormant or nearly so. Under certain 

 conditions, they have the power of entering again into vital activity, 

 losing more or less of their form. Sometimes they generate in their 

 interior large quantities of starch, and become more or less rounded, 

 or under the influence of light they become transformed into chloro- 

 plastids. 

 ^ * Bot. Centralbl., xii. (1882) pp. 175-8. 



