CHAPTER X 

 HEREDITARY DIFFERENCES IN LEAVES 



All leaves develop from a small mound of meristematic cells near the 

 tip of a stem, and all these primordia look much alike. From the leaf 

 primordia of elm trees simple pinnately veined leaves develop, but from 

 the leaf primordia of the sugar maple five-lobed palmately veined leaves 

 develop. The leaves of horse chestnut and buckeye become palmately 

 compound, and those of the ash become pinnately compound. Since 

 these characteristics regularly appear in anv, or all, of the environments 

 in which the trees grow, it is evident that these variations in leaf devel- 

 opment, foiTn, and structure are the result of hereditary differences. 



Heritable variations in leaves have been occurring throughout the 

 millions of years that leaf -bearing plants have existed upon the earth. If 

 one were to begin studying these variations by noting all of the minute 

 heritable differences in the form and size of teeth and lobes on leaf 

 margins, or in the form, number, and arrangement of epidermal hairs on 

 leaves, he would undoubtedlv conclude that heritable variations in leaves 

 are legion. He would probably conclude also that for every one of these 

 variations tliat may be of some advantage to the plant there are hundreds 

 of others that are of no particular value or hami to it. Regardless of their 

 value or lack of value to the plant, all of them are the result of the same 

 general changes in the composition and arrangement of molecules in the 

 hereditary units of matter (genes) in plant cells. "Rain falls alike upon 

 the just and the unjust" because the factors underlying the formation of 

 clouds and rain bear no relation to justice. Similarly, the factors underly- 

 ing the origin of heritable variations bear no relation to their value. Dis- 

 similar variations occur in leaves of plants growing in similar habitats 

 and, conversely, similar variations occur in leaves of plants growing in 

 dissimilar habitats. A few kinds of variations have survived more abun- 

 dantly in some habitats than in others. Many of the heritable variations 

 in leaves are interesting as phenomena of nature. Man makes use of 

 them also as a means of classifying plants, for various sorts of decora- 

 tion, and as a source of certain economic products. We shall not at this 



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