lO CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



The intercellular spaces are separated by a single layer of 

 cells, and the general appearance of the section is much like 

 that of such aquatic Pteridophytes as Marsilia or Azolla. 

 Like these, the bundle is typically concentric. Magnus(i889, 

 p. 215) states, and his statement has not been contradicted, 

 that there is no tracheary tissue present. This is incorrect, 

 at least for N. jlexilis. In longitudinal sections of the 

 younger parts of the stem, spiral or annular tracheids are 

 always found, forming a single line in the axis of the bundle. 

 The thickenings on the walls are formed at an early 

 period, and with the rapid growth of the stem in the 

 lower internodes, these primary tracheids are pulled apart 

 and destroyed, leaving the central canal, which Magnus fig- 

 ures. It may be noted that Caspary(i858,p.5i5) noted traces 

 of these tracheae in this same species, at the base of the 

 funiculus, but apparently overlooked them in the other parts 

 of the plant. It is evident from his account, however, that 

 his investigations were not very extensive, and it is by no 

 means unlikely that further investigation, using microtome- 

 sections of the younger parts of the stem, will reveal the 

 presence of tracheids with normal thickenings, in the other 

 species as well as in N. jlexilis. In common with other 

 submersed aquatics, Naias has the epidermal cells contain- 

 ing chlorophyll, and not noticeably different in form or 

 contents from the outer cells of the cortex. 



II. The Leaves. 



The structure and arrangement of the leaves is very sim- 

 ilar in all species of Naias yet investigated. The base of 

 the leaf is expanded and forms a sheath surrounding the 

 stem more or less completely. In the case of the sterile 

 leaves, i. e., those which develop no axillary products, the 

 base of the leaf envelops the stem completely and one edge 

 overlaps the other, much as is found in the case of the coty- 

 ledon (fig. 80). The marginal cells of the leaf are extended 

 into the brown teeth, which gives the leaves a distinctly 

 serrate outline. Cross-sections of the leaf show a structure 



