340 AMERICAN MIDLAND NATURALIST 



tissue (medullary rays) are so far extended into the cortical region 

 that the interfascicular cambium is nearer to the epidermis than 

 the outermost portions of the phloem, in spite of the fact that the 

 bundles are open (?) collateral of the typical kind and the xylem 

 endarch pentarch. (Figs. 7 and 8). Several layers within the epi- 

 dermis there appear at regular intervals perfectly straight vertical 

 latex tubes and near the fibrovascular bundles a few are found at 

 irregular intervals. The xylem is wedge shaped and has outside of it 

 oval strands of phloem, the latter with stereome in several layers 

 on its outer margin. The protoxylem has the usual annular ducts 

 which gradually pass into spiral and finally scalariform pitted, 

 and the ducts in each bundle arc all arranged in one line or series 

 increasing in size outwards towards the surface of the plant. The 

 centre of the stem or pith is occupied with a large hollow space. 



LEAVES. 



As already intimated there are two kinds of leaves present, 

 the submersed and the emersed. The former are short petioled 

 or nearly sessile and repeatedly palmately dissected into linear 

 divisions which in structure are of the centric type. (Fig. 9). The 

 epidermis which is scarcely differentiated except in size and lack 

 of cellwall markings, bears chlorophyll like the rest of the mesophyll 

 layers. Only the layers of cells immediately bordering on the 

 vascular system are devoid of it. The cells of the mesophyll are 

 polygonal in cross section and their sides are marked transversely 

 with scalariform markings. (Fig. 10). The wood bundle is very 

 simple and varies according to the distance the section is made 

 from the petiole. Fig. 9 shows a cross section made of one of the 

 divisions about the middle of the leaf, several ducts are present 

 as also a latex tube as are phloem elements and wood parenchyma. 

 The ))undles of the larger and lower divisions are more typically 

 open collateral (Fig. 11). Both- xylem and phloem are rather well 

 developed, the former rather more so than were expected in 

 aquatic plants. The epidermis of the submersed leaves is of 

 course totally devoid of stomata and the cells arc somewhat 

 longer than broad. (Fig. 12). The petiole of the aquatic leaf is 

 rather more flattened than any of the divisions but it has only a 

 single larger bundle not essentially diff'erent from the collateral 

 iDundle of the divisions. The phloem strand is rather extensive 



