662 



ECOLOGY 



having adequate room for full development beneath the epidermis. 

 Consequently the epidermis is soon ruptured and the complementary 

 tissue protrudes, forming with the upturned ruptured edges a char- 

 acteristic emergence visible to the unaided eye (figs. 971, 1057- 

 1059); the entire structure, of which the loose complementary tissue 

 forms the most significant part, is known as a lenticel. Though lenticels 

 in some species remain open for many years through the continued 

 formation of complementary cells from phellogen, in most plants the 



cork cylinder eventually be- 

 comes continuous through 

 cork formation underneath 

 the lenticels; often this cork 

 is more permeable than ordi- 

 nary cork, on account of the 

 presence of intercellular air 

 spaces. In many plants 

 closure occurs in the autumn, 

 the lenticels being essentially 

 cut ofiF from the cortex before 

 winter. Such lenticels may 

 remain permanently closed, 

 or the following spring the 

 development of complemen- 

 tary cells underneath the cork 

 may cause it to burst, thus 

 reopening the lenticels to gas 

 exchange. In most woody 

 plants the lenticels are round- 

 ish or slightly elongated struc- 

 tures which disappear after a 

 few years by reason of bark exfoliation, but in the birches and in 

 some cherries they remain for many years, elongating horizontally as 

 the trunk increases in diameter (fig. 974). In some woody stems, as 

 in the grape, lenticels do not occur. 



The causes of lenticel development. — • The development of cork from 

 phellogen is favored by desiccation (p. 706), while the presence of abun- 

 dant water causes the development of a loose tissue with prominent 

 air spaces, the aerenchyma (p. 553). The complementary tissue of lenti- 

 cels, like aerenchyma, is derived from phellogen, and the cause of its de- 



L'lG. 974. — Trunks of the paper birch 

 (Betula alba papyri/era), showing numerous 

 transversely elongated permanent lenticels; 

 Salisbury, Conn. — Photograph by E. W. 



COWLES. 



