THE SEEDLING AND YOUNG PLANT. 79 



ment, however, other dispositions are occasionally met 

 with on the same plant. The young leaves are folded 

 in the bud in such a manner that the two halves of the 

 lamina lie one on the other, the upper surfaces heing 

 in contact (conduplicate vernation), the margins being 

 therefore turned upward. 



In order to understand the structure of the leaf, let 

 us look at a section cut neatly across the midrib and 

 lamina, and examined with the microscope. It is found 

 to consist of three principal parts an epidermis above 

 and below, and all round the margins, and therefore 

 over the whole of the leaf ; this epidermis is, in fact, a 

 continuation of that of the young shoot-axis, and envel- 

 ops the whole of the remaining leaf-tissues. Inside this 

 we have the main mass of the leaf substance called the 

 mesophyll consisting of thin-walled cells arranged in 

 a peculiar manner, and containing (in addition to less 

 obvious structures) large numbers of green chlorophyll 

 corpuscles ; it is the predominance of these corpuscles 

 which causes the leaves to appear uniformly green. 

 Here and there we see vascular bundles, imbedded, as it 

 were, in the mesophyll, cut across in various directions ; 

 and when it is remembered that these vascular bundles 

 constitute the venation of the leaf, this phenomenon is 

 easily explained. 



As we have already seen, the vascular bundles of the 

 venation (Fig. 20) are simply the much-branched and 

 thinned-off upper ends of the vascular bundles from the 

 shoot-axis, the lower ends of which join the vascular sys- 



