FROM SIMPLE LEAVES. 121 



i 



tally than those situated toward its top. All branches natu- 

 rally tend to the light, and the inferior branches necessarily 

 take this direction on account of the overshadowed position 

 which they occupy. If the reader looks at the beech branch 

 figured on page 33, he will see that the upper twigs or branch- 

 lets develop from the main axis of the brancli at. an acuter 

 angle than the lower twigs ; and let him at the same time bear 

 in mind the fact, that the branch repeats, on a smaller scale, 

 the type of the tree itself. 



If we regard the costa or midrib of the leaf as correspond- 

 ing to the stem or principal axis of ramification of the tree, 

 then the first and most prominent sets of fibrous fasciculi, 

 which separate themselves from the midrib or principal 

 fasciculus of the lamina or blade of the leaf, will correspond 

 to the strongest and largest side-branches of the tree ; and it 

 is obvious that in both instances the general figure or outline 

 of both tree and leaf is determined by them. But whilst in 

 a tree the angular inclination of the branches to the stem 

 varies, in a leaf whose lamina or blade is not formed into 

 distinct leaflets there is no such variation of the angular ii^ili-* 

 nation of the principal veins to the costa or midrib, because 

 theveinlets which would seem to correspond to the branchlets 

 of the branch coalesce among themselves. Therefore there 

 can be no change in the angle formed by the principal leaf- 

 veins and costsB, at any epoch in the development of the leaf, 

 in such cases of leaf-organization, whatever correspondence 

 there may be in other respects between their angular direc- 

 tion and that of the leading branches of the tree. 



In compound leaves, however, which make a much nearer apr 

 proach to the organization of the branch in their structure, 

 where the lamina of the principal leaf is developed into one or 

 more generations of leaflets, light causes the angular inclinations 

 of these leaflets or foliaceous hranchlets to vary slowly and con- 

 tinuously at different periods of the day, with reference to the 

 midrib of the leaf, just as the same agent, acting for years on 

 the main branches of a tree, changes their original angular in- 

 clination with reference to the tree stem. Thus the leaves of 

 the American Senna (^Cassia Marihndica) and of the Honey- 



