134 



MOllPHOLOGICAI, DEVELOPMENT. 



are long and slender, and where, consequently, each leaf, ac- 

 cording to its weight, the flexibility and twist of its foot- 

 stalk, and tlie direction of the branch it grows from, falls 

 into some indefinite attitude, the relations are obscured. But 

 where the foot-stalks are stiff, as in the Laurel, it will be 

 found, as before, that from the topmost and upward-growing 

 branches the leaves diverge on all sides ; while the under- 

 most branches, growing out from the shade of those above, 

 have their leaves so turned as to bring them into rows hori- 

 zontally spread out on the two sides of each branch. 



A kindred truth, having like implications, comes into view 

 when we observe the relative sizes of leaves on the same 

 SOS branch, where their sizes differ. 



Fig. 205 represents a branch of a 

 Horse-chesnut, taken from the low- 

 ermost fringe of the tree, where the 

 light has been to a great extent in- 

 tercepted from all but the most pro- 

 truded parts. Beyond the fact that 

 the leaves are bilaterally distributed 

 on this drooping branch, instead of 

 being distributed sjrmmetrically all round, as on one of the 

 ascending shoots, we have here to note the fact that there 

 is unequal development on the upper and lower sides. Each 

 of the compound leaves acquires a foot-stalk and leaflets that 

 are large in proportion to the supply of light ; and hence, as 

 we descend towards the bottom of the tree, the clusters of 

 leaves display increasing contrasts. How marked these con- 

 trasts become will be seen on comparing a and h, which form 

 one pair of leaves that are normally equal, or c and d, which 

 form another pair normally equal. 



Let us not omit to note, while we have this case before us, 

 the proof it affords that these differences of development are 

 in a considerable degree determined by the different con- 

 ditions of the parts after they have been unfolded. Though 

 those inequalities of dimensions whence the differentiations 



