xxviii] PONTEDERIACEAE 343 



flattening and expansion of the petiole in the horizontal plane 

 (Fig. lyo B and C). For, instead of the normal arrangement of 

 bundles, all orientated with the xylem upwards, which we are 

 accustomed to find in laminae, the vascular strands in this case, 

 though in a single series, are orientated, some normally (n.b.\ 

 including the median bundle (m.b.)^t]\t majority inversely (z.(^.), 

 and a few obliquely placed (p.b.). A small part of the transverse 

 section is shown in greater detail in Fig. 1 70 C. In this drawing, 

 the central and largest bundle is seen to be normally orientated, 

 but the bundles on either side of it have the xylem below and 

 phloem above. 



In the heart-shaped ' lamina ' of Heteranthera reniformis^ 

 Ruiz, and Pav., a very similar bundle arrangement is found 

 (Fig. lyoE). Here, only the midrib and main laterals are 

 normally placed, the remaining bundles being inverted. 



The ' lamina ' of Eichhornia speciosa^ Kunth (Fig. 1 70 yf) 

 differs from that of the other members of the family here con- 

 sidered, in its much greater thickness. Inverted bundles occur, 

 not only in the thick basal region in which the transition from 

 petiole to * lamina ' takes place quite gradually, but also near 

 the margin. Here, there is only a single series of vascular 

 strands, among which inversely orientated bundles are very 

 numerous. Some of the lateral veins in the ' lamina ' consist of 

 a single, normally orientated bundle, while others include a pair 

 of bundles, one normal and one inverted. 



Among the Pontederiaceae, we not only find leaves, such as 

 those just described, in which there is a differentiation between 

 petiole and ' lamina,' but others, which are ribbon-like, with no 

 distinction of blade and stalk. For comparison with the more 

 highly differentiated leaves, sections were cut of the ribbon-leaf 

 oi Heteranthera zosteraefolia, Mart. Here the midrib and main 

 laterals proved to be normal, but the others i.e. the majority 

 of the laterals were inverted. Fig. 170 F shows two adjacent 

 bundles orientated in opposite ways. The structure of this 

 ribbon-leaf is closely similar to that of the ' lamina ' in H. reni- 

 formis. 



