42 PHARMACEUTICAL BOTANY 
(d) Tue Pxastips, composed of differentiated protoplasm 
called plastic plasm. They are small, discoidal, spheroidal, 
ellipsoidal, stellate, ribbon-shaped, or variously shaped, dense, 
porous, protoplasmic bodies scattered about in the cytoplasm. 
The plastids may become modified and specialized for definite 
functions depending upon the position of the cells containing 
them in the plant and the external conditions to which the cells 
are exposed. Sometimes, as in the cells of lower plants 
like the Spirogyra, plastids are large and are then called 
CHROMATOPHORES. 
In the meristem cells of root and stem tips of plants the plastids 
are very small, colorless and usually found around the nucleus. 
These minute plastids are termed primordia or proplastids. As the 
meristematic cells enlarge and undergo differentiation, the 
proplastids increase in size and develop into leucoplastids, chromo- 
plastids and chloroplastids. 
According to the position of the cells in which plastids occur 
and the work they perform, they differ in color, viz.: _ 
Leucop.astips are colorless plastids found in the under- 
ground portions of a plant and also in seeds, and other regions 
given up to the storage of starch. Their function is to build up 
reserve foods like reserve starch and oils from sugar and other 
carbohydrates. Leucoplastids may become transformed into 
chloroplastids upon exposure of cells containing them to light 
through the formation within them of chlorophyll and related 
pigments. They are only evident after properly fixing and 
staining cells containing them. When leucoplastids build up 
and store starch they are sometimes called amyloplastids; when 
they build up and store oils they are called elatoplastids. 
CHLorop.astips are plastids found in cells exposed to light 
and contain the green pigments chlorophyll a and chlorophyll b 
together with two yellow pigments known as carotin and xantho- 
phyll. All green parts of plants contain these bodies. Chloro- 
plastids may develop into chromoplastids, as in the ripening of 
many green fruits. The fully developed chloroplastid is capable 
of division by constriction. 
CHROMOPLASTIDs are plastids found in cells independent of 
their relation to light or darkness and contain a yellow, orange 
