LECT. VII.] ANATOMY OF STEMS. 321 



outwards by the increase of the diameter of the 

 stem ; and the accumulation of such layers forms 

 the rugged surfaces of stems, as for example of 

 the Elm, the Oak, and the majority of trees *. 

 In the greater number of instances it cracks verti- 

 cally, and is pushed outwards with a portion of the 

 cellular integument by the new epidermis, which 

 can be brought into view by removing these 

 rugged portions. In others it splits horizontally, 

 and the new cuticle is formed immediately under 

 the old, which, after a time, detaches itself in 

 fragments ; or, there is a succession of cuticles, 

 which, although one is formed every year, yet 

 do not separate annually, but occasionally only, 

 in multiplied layers, that can, however, be readily 

 detached from each other, as in the Currant and 

 the Paper Birch. Some trees, the Plane for ex- 

 ample, annually throw off the cuticle at once, in 

 large flakes ; and in this respect, such plants re- 

 semble those reptiles that cast their skins or their 

 crusts, as the snake, the spider, and the lobster. 



2. The Cellular integument. On carefully 

 raising the cuticle of the young shoot of the Horse 

 Chesnut, we find under it a cellular layer ; which, 

 in a transverse section of the stem placed under 



* In Plate 6, fig. 10. a. represents a microscopic view of the 

 various layers which form the rough cuticle of an old stem 

 of the Lilac, Syringa imlgaris ; the innermost only of which 

 retains its vitality. 



VOL. I. Y 



