VEGETABLE CYTOLOGY 



6 9 



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 

 trom 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. 



