60 



DESCRIPTIVE BOTANY. 



chyma is so little developed, that the leaf appears to 

 consist entirely of the veins, and resembles those skeletons 

 of leaves which are sometimes pre- 

 pared by maceration in water. A 

 large proportion of trees produce 

 fresh leaves in the spring, which 

 afterwards fall in the autumn ; 

 such are termed " deciduous," in 

 contradistinction to " evergreens," 

 which are never entirely divest- 

 ed of leaves. No plant, how- 

 ever, retains its leaves for more 

 than two or three years ; but as 

 the leaf-fall in evergreens is par- 

 tial, consisting perhaps of one 

 half or one third at a time, there 

 are always a sufficient number left 

 on the tree, to keep it clothed with 

 perpetual verdure. 



In succulent plants, the ves- 

 sels which quit the stem to form the leaf, diverge in 

 different planes, and the leaves in 

 consequence consist of solid fleshy 

 masses of cylindrical and other solid 

 forms, instead of flattened laminae. 



The complete leaf consists of 

 two parts : the leaf- stalk, or 

 " petiole ;" and the expansion, 

 or " limb." There is often an ^^-t CV 

 alteration in the colour and tex- 

 ture of the petiole at the point 

 where it is attached to the branch, 

 and sometimes a slightly swollen 

 protuberance. This is termed an 

 " articulation ; " and it is at that 

 part that a disunion takes place at the period of 

 leaf-fall, and a " scar " is left upon the stem. But 

 where no articulation exists, the withered petiole re- 



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