CHAPTER V 

 CHEMISTRY OF CHLOROPHYLL: THE CAROTINOIDS 



I will allow there is much to be said for yellow and green. 



— Eugene Field. 



Plastids. — The chlorophyll in most plants, as can be easily 

 seen by examining a leaf under the microscope, is not distributed 

 evenly through the protoplasm but is contained in special struc- 

 tures called plastids. These green bodies which contain the chloro- 

 phyll are of various shapes and sizes. In many of the algae they 

 are ribbon- (Spirogyra), star- (Zygnema), or cup'(Chlamydomonas) 

 shaped, but in the higher plants the ordinary shape is that of an 

 elongated hollow spheroid with a central vacuole. As the cyto- 

 plasm moves about the cell, it may carry with it the chloroplastids, 

 which in this case serve as an index to the movement. 



Origin of Plastids. — The plastids do not originate de novo, i. e., 

 out of nonplastid material, but come from preexisting plastids or 

 their forerunners. In some cases they have been seen to arise from 

 the division of plastids and, as a result of these observations, it 

 has been proposed that they originated in the cells of plants as 

 independent one-celled organisms which have been held there and 

 imprisoned. The work of Lewitski (1910), Zirkle (1926), and 

 others, however, has led to the conclusion that the plastids are 

 not imprisoned algae but that they arise from minute bodies in the 

 protoplasm not seen by ordinary methods of staining which belong 

 to the class of bodies called chondriosomes. These structures are 

 essential parts of the protoplasm which by their division and in- 

 crease in size may give rise to the plastids. 



Relation of Plastid and Pigment.— The plastid seems to be 

 a spongelike matrix of protoplasm in the meshes of which is the 

 chlorophyll. The condition of the pigment within the plastid has 

 been the subject of much dispute. Some workers have expressed 

 the view that it is distributed throughout the groundwork of the 

 plastid as discrete colloidal particles; but most of the evidence 

 indicates that it is dissolved in lipoids (Chap. XIII) in the plastids. 

 The followers of both theories assume that the chlorophyll is in a 



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