THE GREEN LEAF. 57 



stems differ from the stems of herbs only in the thickness 

 of their cell-walls and the absence of livin<,r matter in the 

 woody tissues. Accordingly, trees appear in the most 

 widely different families of plants ; and sometimes they 

 are closely related to very small and weedy types, as 

 amonrr the roses, which vary in stature from little creep- 

 ing herbs like the wild strawberry to tall trunks like the 

 pear-tree. Wide as the difference seems to us, it is but 

 a slight one in reality : a tree is only a herb which has 

 prospered best by growing stiff and perennial, and so 

 has acquired the habit of making its stem veiy stout and 

 hard, to resist the greater mechanical strains that will 

 now be brought against it by wind and weather. In all 

 essential points each tree still preserves all the main 

 features of the family to which it happens to belong. 



