PLASTIDS 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 tothe chlorophyll. Thus 
in the Red Seaweeds (Rhodophyceae) the 
chloroplasts are usually red, in the Brown 
Algae (Phaeophyceae) they are brown, in 
some Myxophyceae the chloroplasts are 
bluish green, etc. Chlorophyll proper is 
a bluish green, apparently somewhat oily 5,, 3 plastias 
substance, probably contained in inter- (chloroplasts) in 
stices of the chloroplast. It is soluble 
in alcohol, by means of which it can be removed, leay- 
ing the chloroplast colorless. In addition to chlorophyll 
most chloroplasts contain an orange yellow pigment, to 
which the name xanthophyll 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. 
