THE CELL. 15 



starch-builder (amyloplastid) is colorless and forms starch within or 

 upon it in the manner described. 



Leucoplastids are very unstable and easily destroyed, hence not 

 readily demonstrable. In the living cell they must be rapidly 

 acted upon by certain reagents, as for example an aqueous iodine 

 solution. Their form is usually spherical, sometimes elongated or 

 spindle shaped owing to the presence of crystalloids. 



The chromoplastids form the coloring substances of the variously 

 colored flower and fruit parts. One fact is to be remembered, 

 namely, that the red, blue, and violet colors are often due to sub- 

 stances in solution in the cell-sap. Yellow chromoplastids, for 

 example, are found in the cells of the beet. The form of the 

 chromoplastids varies ; they may be spheroidal, disk-like, radiate, or 

 elongated. 



At this point it is well to make a few statements concerning (1) 

 metamorphosis; (2) the destruction of chlorophyll (SACHS, G. KRAUS). 



1. Chlorophyll bodies may be converted into red chromoplastids, 

 thus causing the red coloring of fruits. This has been demon- 

 strated in many instances. The red detected in the winter color- 

 ation of evergreen leaves (Conifers, Buxus) disappears in the spring. 

 In these plants the chlorophyll bodies are not entirely destroyed ; 

 they lose only a portion of the green coloring matter, while carmine- 

 red drops appear which again disappear in the early spring. 



2. On the other hand the autumn coloration of falling leaves, 

 the yellowness of straw, the change in color of dying plants or parts 

 of plants, is due to the destruction of the chlorophyll.. The entire 

 plasm and the chlorophyll bodies of falling leaves enter into dis- 

 solution and the important constituents pass into the more persist- 

 ent parts. A yellow coloring matter (xanthophyll) remains in the 

 leaves in the form of small granules. In the case of the red coloring 

 of falling leaves there is in addition to these yellow granules a red 

 cell-sap (grape). 



There is still another plasmic structure to be mentioned which 

 the more delicate microscopic manipulations have % brought to our 

 notice, namely, the so-called " starch or amylum clusters " or 

 " pyrenoids." In composition they resemble most nearly the 

 nucleoli or nucleus. They are found almost exclusively among the 

 algae, where they usually occur within the chlorophyll bodies, evi- 

 dently constituting centers of starch-formation. As a rule they are 

 enveloped by numerous starch-granules, hence the name. 



