PLASTID8 11 



plastids are formed only from the division of old plastids 

 into two parts. They are difficultly visible in some plant 

 cells, e.g. in the small rapidly dividing meristem cells at 

 the growing points of a plant, and are entirely lacking in 

 some great groups of plants, viz. the bacteria and fungi. 

 19. Chloroplasts are plastids containing chlorophyll. 

 Ordinarily they are green, from the color of the chloro- 

 phyll itself, but in some groups of plants the green color 

 is masked by the presence of other pigments in the chloro- 

 plasts in addition to the chlorophyll. Thus 

 in the Red Seaweeds (Rhodophyceae) the 

 chloroplasts are usually red, in the Brown 

 Algae (Phaeophyceae) they are brown, in 

 some ]\Iyxophyceae the chloroplasts are 

 bluish green, etc. Chlorophyll proper is 

 a bluish green, apparently somewhat oily p^^ 3— piistida 

 substance, probably contained in inter- (cMoropiasts) in a 

 stices of the chloroplast. It is soluble 

 in alcohol, by means of which it can be removed, leav- 

 ing the chloroplast colorless. In addition to chlorophjdl 

 most chloroplasts contain an orange yellow pigment, to 

 which the name xanthophjdl is often applied. It ap- 

 pears to be a form of carotin. The mixture of these 

 two gives the grass-green color to the chloroplast. With 

 rare exceptions chlorophyll is not produced in the ab- 

 sence of light. It usually disappears in prolonged dark- 

 ness, leaving the chloroplast stained yellow with xantho- 

 phyll or colorless. In many of the lower plants the 

 chloroplasts are of various shapes, often being star-, 

 band-, plate-, or even net-shaped. In the higher plants 

 they are mostly more or less disk shaped. In some of 

 the liverworts and many of the algae they contain one 

 or more highly refractive bodies, called pyrenoids, which 

 are probably crystals of some albuminous substance. 



