64 MORPHOLOGICAL DEVELOPMENT. 



which are very variable, not only in different plants but in 

 the same plant, there results a modification like that de- 

 lineated in Fig. 96. And then, in such forms as Fig. 97, there 

 is shown the arrangement that arises when, by more rapid 

 development of the proximal end of the mid-rib, the distal 



part of the foliar surface is separated from the part which 

 embraces the axis: the wings of the mid-rib still serving, 

 however, to connect the two portions of the foliar surface. 

 Such a separation is, as pointed out in 188, an habitual 

 occurrence; and in some compound leaves, an actual tearing 

 of the inter-venous tissue is caused by extra growth of the 

 mid-rib. Modifications like this, and the further one in Fig. 

 98, we may expect to be established by survival of the fittest, 

 among those plants which produce considerable masses of 

 leaves; since the development of mid-ribs into footstalks, by 

 throwing the leaves further away from the axes, will diminish 

 the shading of the leaves, one by another. And then, among 

 plants of bushy growth, in which the assimilating surfaces 

 become still more liable to intercept one another's light, 

 natural selection will continue to give an advantage to those 

 which carry their assimilating surfaces at the ends of the 

 petioles, and do not develop assimilating surfaces close to the 

 axis, where they are most shaded. Whence will result a dis- 

 appearance of the stipules and the foliar fringes of the mid- 

 ribs; ending in the production of the ordinary stalked leaf, 

 Fig. 99, which is characteristic of trees. Meanwhile, the axis 

 thickens in proportion to the number of leaves it has to 

 carry, and to put in communication with the roots; and so 



