The Morphology of Ruppia Maritima. 77 



layer are much larger than those of the epidermis, but partake 

 somewhat of the nature of the latter in their slightly thickened walls. 

 Toward the outer part of this cortical region intercellular spaces 

 rarely occur: instead the triangular prismatic areas which the spaces 

 would occupy are filled with thickening and probably supplement 

 the firmness imparted to the stem by the epidermis. 



It is in this outer portion of the cortex that two small vascular 

 bundles occur, which may be termed cortical bundles (PI. I, figs. 4, 

 5, ch). An account of these will be given in connection with the 

 vascular system. 



Toward the interior of this zone the cells increase in size, and 

 intercellular spaces begin to appear. 



(b) Middle cortex. 



The most striking feature of the middle area of the cortex is the 

 ring of large air spaces or lacunae (PI. I, figs. 4, 5, mc and la\ PI. Ill, 

 fig. 7). These originate schizogenously by a splitting apart of the 

 walls of adjacent cells when these are in a very young stage, and 

 subsequently they become much enlarged. A ring of from twelve 

 to eighteen is disposed in a fairly symmetrical way in this central 

 part of the cortex. As a rule somewhat elongated radially, they 

 are separated from each other laterally by a single column of cells. 

 In length up and down the stem they extend from node to node, 

 no diaphragms being stretched across as in the leaf lacunae. Com- 

 munication through the node is maintained by means of small inter- 

 cellular spaces. 



Between the lacunae and the small intercellular spaces occurring 

 near the inner and outer limits of the cortex lie many air spaces of 

 intermediate size, so that although the large lacunae appear quite 

 distinct on account of their large size and fairly regular arrangement, 

 yet gradations exist between them and the small triangular inter- 

 cellular spaces (PL I, fig. 5). 



(c) Inner cortex. 



The interior zone of the cortex, terminating with the endodermis 

 (PI. I, fig. 5, end), is a good deal similar to the outer zone. The 

 cells and intercellular spaces gradually decrease in size, and the 

 latter also in frequency, toward the endodermis, while the cell walls 

 gradually increase in thickness. 



A striking peculiarity of the cells of this inner cortical zone is 

 their chlorophyll content, which is very noticeable in freehand 

 sections from fresh stems. It was definitely ascertained that the 

 chlorophyll grains belong to these cells and were not drawn inward 

 from the epidermis by the razor. 



