BY THE SELECTION OF SOMATIC VARIATIONS. 65 



In Cokus the development of patterns is considered by the writer 

 to be due largely to cellular and tissue interactions influencing general 

 and metidentical qualities with results quite analogous to the Liesegang 

 phenomena. Changes involving red are on this basis rather simple 

 cases of readjustment influencing the total amount of pigment produced 

 and the distribution in centers or areas. The ability to produce the 

 different chemical substances concerned with the final development of 

 the red pigmentation is assumed to be a general property or potentiality 

 of all Coleus cells. The assembling of all the products necessary for 

 the final stages in its development, however, are determined by the 

 amounts produced and their flow to centers of activity and interaction. 

 The development of red, especially in the subepidermal tissues, indi- 

 cates that this is the case and suggests strongly that, as shown by 

 Overton (1899), changes in the amounts of red pigmentation may be 

 closely related to changes in the sugar-content of the sap. In Coleus, 

 however, it is clear that such changes arise quite spontaneously in the 

 cells and tissues. 



In respect to the development of green and yellow in particular cells, 

 the processes seem to be antagonistic. Plastids are present in both 

 green and yellow cells, but in yellow cells they are fewer in number, 

 smaller in size, and somewhat distorted in shape. The green and 

 yellow cells are subepidermal, extending from upper to lower epidermis. 

 In a pure-green leaf all these cells remain green. In the most extreme 

 cases of yellow development nearly all the cells fail in the production of 

 chlorophyl. The different patterns result from variations in the 

 relative number of green and yellow cells and in the grouping of the 

 cells of like color. In some a green field is blotched with island-like 

 areas of yellow cells, in others the central area of green is bordered 

 by an irregular band of yellow, and again the yellow may be situated in 

 the center with a green band at the border. 



In a leaf with blotched or banded green-yellow patterns the inter- 

 mingled areas of green and yellow cells indicate quite clearly that both 

 types of cells are derived from the same cells in the growing-points. 

 In the development of leaves it can be observed that while the yellow 

 areas are in evidence when the young leaves unfold, the yellow seems 

 tinged with green, and that as the leaf grows the yellow becomes 

 more intense. Furthermore, the yellow bleaches until in old leaves 

 the yellow areas change to a pale yellow or white, while the green areas 

 are still bright. As a leaf dies the green areas become pale, greenish 

 yellow. These observations indicate that many of the cells which 

 later become yellow are actually green at first. 



The fluctuations that appear substantiate this view. A plant with 

 yellow-bordered leaves may produce, especially in winter, new leaves 

 entirely green, and thus possess for some time old leaves of green-yellow- 

 red blotched pattern and younger leaves of green-red blotched pattern. 



