xiv] LACUNAE IN PITH AND CORTEX 185 



the air spaces are schizogenous, they may be arranged in the 

 form of a single ring (e.g. Myriophyllum, Fig. 1 16, p. 179), or a 

 number of rings may occur, giving a lace-like appearance to the 

 stem, when seen in transverse section (e.g. Hippuris). The 

 development of the air spaces in the cortex of Hippuris vu/garis 1 

 is illustrated by Fig. 120 A and B. The Water Crowfoot 

 forms a transition to those plants in which the air spaces are 

 lysigenous, for, in the young stem, irregularly placed schizo- 

 genous air spaces occur, especially in the pith (Fig. 1 1 3, p. 1 76), 



FIG. 1 20. Hippuris vulgaris, L, Parts of transverse sections through a younger 

 stem (A) and an older stem (B) showing the origin of the cortical lacunae. 



[Barratt, K. (1916).] 



while, in the older stem, the whole of the central parenchyma 

 becomes torn and destroyed, leaving a large axial lacuna. 

 Pep Us Portu/a 2 is an example of a plant whose air spaces are 

 mainly lysigenous. In transverse sections of the internodes, 

 four such spaces are visible, each containing the torn remains 

 of cells. 



The aerating system of the roots of aquatics is to be found 

 in the cortex. In some cases, e.g. Vallisneria^ the intercellular 

 spaces may be small, but more frequently they are of con- 

 spicuous size, and arranged with a regularity that gives a notable 



1 Barratt, K. (1916). 2 Schenck, H. (1886). 



