CHAPTER III. — ORGANS OF PLANTS. 



I 9 I 



occur, a continuous covering, though minute elevations may be 

 discernible here and there on the surface, even before t'.ie close of 

 the first year's growth. This is due to the growth of cells at 

 certain points beneath. During the second year, the continuity is 

 ruptured by Assuring at these points, and the cells from beneath 

 protrude, giving to the surface a freckled or spotted appearance. 

 These spots are technically called lenticels, and their character 

 will be understood by reference to Fig. 453. 



Sooner or later, however, a corky layer ox periderm is formed 

 by the division of cells in a tangential direction. This periderm 

 may be formed superficially either by the division of the cells of 





Fig. 453. — Transverse section through lenticel of the White Birch. s, stoma: /, cells of 

 the lenticel which, by their rapid increase by fission in the inner layer, have caused an 

 elevation of the epidermis, but have not yet burst through it; e, epidermis. Magnified 

 about 280 diameters. After DeBary. 



the epidermis itself, as in the Willow, Apple and Oleander, or, as 

 is more commonly the case, by the division either of the paren- 

 chyma cells in immediate contact with the epidermis, or of those 

 forming the second or third layer beneath it. In this case, the 

 epidermis and the one or two layers of parenchyma that are thus 

 cut off from supplies of nutriment by the formation of cork 

 interior to them, die and disappear, and the periderm, including 

 the cork and the merismatic layers called phellogen, in which 

 new cork cells are formed, now constitues the epiphloeum. It 

 increases in thickness by the formation of new cells in the phel- 

 logen by tangential division, but at intervals rows of cells divide 

 in a radial direction thus enabling the layer to keep pace, for a 

 time, at least, with the growth of the stem in circumference. 

 Examples of plants that form a periderm of this character are 

 the Beech, Chestnut, Hazel and Bass-wood. 



