CHAP. I.] PLANT ARCHITECTURE. 41 



surface of the leaf, and about 3,000 on the upper ; in 

 the cherry laurel, about 90,000 in a square inch of the 

 under surface of the leaf, and none on the upper surface. 

 When leaves grow erect, stomata are usually about 

 equally abundant on the upper and under surface ; but 

 when growing horizontally, with one surface to the 

 earth — the lower — and the other to the sky — the upper — 

 stoma;ta are usually much more numerous on the under 

 surface. Owing to the disappearance of the greater 

 portion of the protoplasm from epidermal cells at an 

 early period, no secondary growth or cell- division can 

 take place; hence, as already stated, typical epidermis is 

 met with as the permanent covering of leaves, for the 

 full development of a leaf takes place rapidly and is 

 contemporaneous with that of the epidermis which is 

 formed from its superficial layer of cells, and however 

 long a leaf may continue to live, there is no secondary 

 growth or increase in size. Young twigs and shoots are 

 protected with epidermis during the first year, but at 

 the commencement of the second year's growth, when 

 the twig begins to increase in thickness, the epidermis, 

 unable to grow and keep pace with the expansion of the 

 twig, is ruptured and thrown off', its protective function 

 being superseded by the formation of a waterproof 

 structure called periderm, which is formed from the 

 superficial cells of the twig, but differs from epidermis in 

 not being composed of one layer of cells only, the absence 

 of stomata, and more especially by a provision for the 

 increase in the number of cells by means of which the 

 periderm keeps pace with the increase in thickness of 

 the twig. 



When cells by repeated bipartition produce other 



