PL AST IDS d 



not been allowed to dry out are suitable for a study of the stream- 

 ing movement of the cytoplasm as well as changes in the relative 

 position of the nucleus in the hair-cells. 



III. Plastids. — Specialized cytoplasmic bodies known as plas- 

 tids are commonly found in many types of plant cells. Recent 

 studies (cf. Weier 1938) emphasize the many unsolved problems 

 concerning the origin, fundamental structure and activity of 

 plastids, in particular of the photosynthetic chloroplast. 



Obtain a leaf from the outer region of the terminal bud of 

 Elodea and mount it carefully in a drop of water. Examine this 

 leaf first under low magnification, noting that the thin lateral 

 flaps at either side of the "midrib" are composed of only two 

 layers of essentially similar cells. In short, there is no mesophyll 

 as distinct from an epidermis. Study a number of cells in this 

 leaf at varying depths of focus under high magnification. Note 

 that all cells contain numerous small discoid chloroplasts which 

 typically are in a iieripheral position. Many cells, at least in 

 healthy leaves, will exhibit cytoplasmic streaming. Careful ex- 

 amination will show that while the chloroplasts are non-motile in 

 themselves, they are passively carried in a clockwise or counter- 

 clockwise direction by the circulating cytoplasm. Often the 

 chloroplasts are so numerous in a cell that they mask the nucleus. 

 The latter can be more easily observed in the more transparent 

 teeth-like cells which occur irregularly at the margins of the 

 leaf. 



Another extremely common type of plastid is the chromoplast 

 which frequently produces the red, orange, or yellow color of 

 petals, fruits and certain roots. {Note: Since similar colors may 

 be produced by anthocyanin pigments in the vacuome, the basis 

 for color in each instance can only be determined by first hand 

 investigation.) The role of chromoplasts in general is obscure 

 and demands further study. 



Prepare water mounts of tomato fruit, carrot root, asparagus 

 and rose "hips," and examine them under low magnification. 

 Note the wide variation in size and shape of the chromoplasts. 

 According to Eames and MacDaniels (1925, p. l.S) the angular 

 forms of the chromoplasts in the carrot (Daucus) "are largely 

 due to the presence of crystals of coloring matter." 



