VEGETABLE CYTOLOGY 69 
Sometimes, as in the cells of lower plants like the Spirogyra, plastids 
are large and are then called chromatophores. 
According to the position of the cells in which plastids occur and 
the work they perform, they differ in color, viz: 
Leucoplastids are colorless plastids found in the underground 
Fig. 33. — A, embryonic cells from onion root tip; d, plasmatic membrane; c, cy¬ 
toplasm; a, nuclear membrane enclosing the thread-like nuclear reticulum; b, 
nucleolus; e, plastids (black dots scattered about). B, older cells farther back 
from the root tip. The cytoplasm is becoming vacuolate; /, vacuole. C, a cell 
from the epidermis of the mid-rib of Tradescantia zebrina, in its natural condi¬ 
tion on the right, and plasmolyzed by a salt solution on the left; g, space left by 
the recedence of the cytoplasm from the wall; the plasma membrane can now be 
seen as a delicate membrane bounding the shrunken protoplast. All highly mag¬ 
nified. (Stevens.) 
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 starch 
from sugar and other carbohydrates as well as to change the reserve 
starch back into sugar when it is needed for the growth of the plant. 
