The Morphology of Ruppia Moritima. 



75 



2. Stem Structures in the Vegetative Region. 



a. Upright Stem. 



In its internal structure the stem is remarkable in many respects, 

 but chiefly because of the reduction and consolidation of the vas- 

 cular system into what may be termed a single axial vascular bundle, 

 if we except the two minute bundles situated in the cortex. The 

 morphogenesis of this reduced structure, as well as the structure 

 of the whole stem as regards its ad- 

 aptability to its environment, will be 

 considered more in detail later. 



The epidermis surrounds a large 

 zone of cortical parenchyma cells 

 with a ring of lacunae in their midst ; 

 these cortical cells adjoin an endo- 

 dermis, which encloses the axial 

 vascular area (PI. I, figs. 4—5 ; PL III, 

 figs. 7, 8 and 9). The four parts 

 — epidermis, cortex, endodermis and 

 vascular system will now be de- 

 scribed in order. 



(1) Epidermis. 

 The epidermal cells are small in 



comparison with the cortical par- 



enchj'ma cells and much smaller 



than the epidermal cells of the root 



(PI. I, fig. 5). Their walls are thin, 



and yet thicker than the walls of 



the interior cells, the free wall being slightly thicker than the others. 



Occasionall}' these cells contain a small amount of chlorophyll. 



As might be expected from a comparison with other submerged 

 plants already investigated (Schenck, 1886), no stomata occur throiigh- 

 out the plant. 



In the epidermis, rather regularly distributed over the stem, but 

 especially abundant in the region of the nodes, are secretion cells, 

 which are, however, more numerous in the leaf, and will be de- 

 scribed more in detail there (see p. 90). These cells occur scattered 

 here and there also in the cortex, and some may be seen in the 

 axial vascular bundle. 



(2) Cortex. 



The typical cortical parenchyma cells are large in comparison with 

 the cells of the epidermis, endodermis and vascular tissue ; thej' are 

 Trans. Conn. Acad., Vol. XIV^^ 6 December, 1908., 



Figui-e 2. — Longitudinal section 

 tlu'ough. growing point of stem, 

 showing origin of youngest leaf 

 primordium in the periblem ; d^ 

 dermatogen ; pb^ periblem ; pi. ple- 

 rome ; Ll. Z-''^, older leaves. 

 X 210. 



