xiv] SECONDARY AERENCHYMA 187 



be seen that a series of plates, side by side, elongating indepen- 

 dently, and at the same time prevented from stretching to their 

 full length, will naturally become detached from one another 

 at certain points, leaving spaces between. The result of these 

 processes is that the middle cortex, as seen in transverse 

 section, consists of radial plates of cells, like the spokes of a 

 wheel, in contact or separated by lacunae, whereas in tangential 

 section the plates are found to meet their neighbours at intervals, 

 so as to form a network. 



In some plants, e.g. Myriophyllum and Callitriche verna^, the 

 air spaces in the root cortex may be increased by the replace- 

 ment of small schizogenous air spaces by large cavities of a 

 partially lysigenous nature, due to the disruption of the septa. 



Remarkable as is the aerating system developed in the pri- 

 mary tissues, that formed in the course of secondary growth is 

 often even more conspicuous. This secondary aerating system, 

 or aerenchyma, arises in some cases from a phellogen, in others 

 from a typical cambium. We will first consider that which is 

 produced by a phellogen, and may be regarded as a special 

 modification of an ordinary periderm. It is well known that in 

 land plants the impervious corky mantle, which so often covers 

 the older parts, is interrupted at intervals by lenticels, or patches 

 of powdery cork, in which the cells are slow in becoming 

 suberised, and are separated by intercellular spaces, instead of 

 being closely applied to one another as in normal periderm. 

 These lenticels form a channel by which gaseous exchange takes 

 place between the atmosphere and the interior of the plant. 

 We have thus, in the lenticel tissue, an example of an aeren- 

 chyma formed on a small scale by ordinary terrestrial plants, 

 and, moreover, this aerenchyma has a tendency to become 

 hypertrophied when the plant is submerged. The case has been 

 described, for instance, of a Poplar branch which had been a long 

 time under water, and in which masses of whitish tissue pro- 

 truded from the surface in many places. On examination these 

 protrusions proved to be due to a great development of the 



iSchenck, H. (1886). 



