The Plant Cell 21 



in form. It is differentiated from the cytoplasm by a 

 distinct membrane, but the minute structure is only ap- 

 parent by the use of staining methods. It shows usually 

 a strong affinity for many stains, and its parts may react 

 in a differential manner. In the growing nucleus there is 

 usually a refractive reticulum staining rather lightly in 

 general, but deeply at certain points, or angles, where there 

 is or seems to be an aggregation of chromatic substance. 

 There is also present upon the reticulum at least one 

 nucleolus. This latter is most evident by staining, but 

 in the unstained nucleus it is strongly refractive, and often 

 serves to locate the nucleus. 



Nucleus and cytoplasm are interdependent, and few 

 cells are long functional in which either of these parts is 

 killed, or from which either is removed. 



14. Plastids. In addition to cytoplasm and nucleus 

 the other protoplasmic organs of the cell requiring brief 

 mention at this point are dense bodies termed plastids, 

 usually disposed in the parietal protoplasm of the cell. 

 In the higher plants they are usually spheroidal or ellip- 

 soidal in form. Of these there are three types : (1) chlo- 

 roplasts, containing the pigment chlorophyll, to which is 

 due the green color of plants, essential, as shown later, 

 in the manufacture of organic food-material in the green 

 plant; (2) leucoplasts or amyloplasts, those starch-form- 

 ing plastids contained in subterranean or other organs of 

 the plant receiving no light, plastids, nevertheless, 

 which are able, when exposed to light, to develop into chlo- 

 roplasts ; and (3) chromoplasts, plastids of various colors, 

 generally yellowish to red, sometimes crystalline in form, 

 from the presence of albuminous crystals, or from the crys 



