No. 449-] STUDIES OA THE PLANT CELL. 375 



form, which are called chloroplasts when green, chromoplasts 

 when the color is other than green or leucoplasts if colorless. 

 These plastids are without doubt derived from the more primitive 

 chromatophores. 



The colors of chromatophores are various. They are believed 

 always to contain some chlorophyll but this green is frequently so 

 completely masked by other pigments that its presence can only 

 be determined when the additional coloring matters have been 

 extracted. Chloroplasts are universally green except when they 

 may be changing into chromoplasts. Chromoplasts generally 

 take their tint from the predominance of other strong pigments 

 in addition to chlorophyll as phycoerythrin in the red and phyco- 

 phsein in the brown algae. But chromoplasts may be derived 

 from chloroplasts whose green has largely or wholly disap- 

 peared leaving other pigments present as the yellow, xanthophyll, 

 or the orange red, carotin. 



The remaining plastids, leucoplasts, are devoid of color and 

 are found in embryonic regions such as eggs, growing points, 

 and in the various tissues of seeds, underground organs and 

 other structures where the cells are largely or wholly removed 

 from sunlight. The leucoplasts may become green upon expo- 

 sure to light thus changing into chloroplasts. They are respon- 

 sible for the secretion of reserve starch in many structures (e. g. 

 potato) and in consequence have been called ^myloplasts. 



Leucoplasts, chloroplasts and chromoplasts are morphologically 

 the same structures. It is well known that they may pass one 

 into the other in the order indicated and that chloroplasts and 

 chromoplasts may lose their color and become leucoplasts. It is 

 generally believed that plastids are not formed de novo. They 

 divide by constriction and thus multiplying are passed on from 

 cell to cell and it is believed from generation to generation. 

 They are therefore usually ranked as permanent organs of the 

 cell. However, it is but fair to call attention to the fact that 

 there are some serious difficulties in the way of a complete 

 acceptation of these views. 



The protoplasmic structure of the plastids of higher plants 

 is rather simple while that of the chromatophores in algae is 

 more complex since they contain a special organ termed the 



