THE SEEDLING AND YOUNG PLANT 79 



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 

 being in contact (conduplicate vernation), the margins 

 being therefore turned upwards. 



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 

 envelops the whole of the remaining leaf-tissues. In- 

 side 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, embedded, 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- 

 tem of the latter lower down. Now the next point to be 



