THE VERBENA FAMILY 111 



also show a beautiful nehvork of veins or ribs. We can guess 

 why the ribs of these large leaves are so strong. It is, no doubt, 

 because thej) are so large that thev require also a strong frame- 

 work to support them. 



The ribs have also other functions, namely, on the one hand 

 to carry the sap, which ascends from the roots through the trunk 

 aud the branches, to the leaf and distribute it over its whole 

 blade, and on the other hand, to receive the products of assimi- 

 lation (starch, etc.) from the green tissue of the leaf, and to carry 

 them down through the petiole to the trunk where they are dis- 

 posed of in such a way as is good for the general growth of the 

 plant. Thus, there is a How of sap ascending the tree to the 

 extremest parts of its leaves, and another descending from these 

 cells to the trunk. And for these two streams in opposite direc- 

 tions there are also different groups of vessels in every leaf-rib 

 and in the leaf-stalk. 



In the dry season the leaves fall down. This also is beneficial 

 to the tree. For the leaves have such a large surface that the 

 tree would lose too much moisture by them, and would wither 

 and die, if they remained on the branches. (See page 52.) 



P'reparation for tlie fall of the leaves is made long before they 

 actually fall. A fine line or ridge may be traced just below the 

 junction of the leaf with the stem. This dark line is in reality 

 a thin, transverse layer of cork, which, when the leaves have done 

 their work during the year, taking in stores of nourishment for 

 the benefit of the tree, grows and so detaches the leaf from the 

 stem. It is interesting to note also that the starch which the 

 leaves have been making during their life-period, is not lost 

 with them, but is transferred to the stem previous to their fall, 

 and chiefly stored up just below the base of the leaf-stalk, so as 

 to afford nourishment to the bud Avhich is found in the axil of 

 every leaf. 



When they fall beneath the tree, they become leaf-mould 

 (humus), which, in its turn, when fully decayed, restores to the 

 soil a large proportion of the minerals taken from it by the roots 

 of the tree. Note also when the leaves fall and decay, how the 

 soft part between the veins rots first, leaving a beautiful skeleton 



