v] ANATOMY OF THE PONDWEEDS 65 



The tendency to condensation and simplification of the stem 

 stele, which is so well illustrated among the Potamogetons *, is, 

 as we shall see in Chapter xm, a characteristic of many aquatics. 

 The stem of the Pondweeds is, however, peculiar in that the 

 bundles are not confined to the central cylinder. In some spe- 

 cies there is a complicated system of cortical strands, occurring 

 at the intersection of the diaphragms separating the lacunae. 

 These cortical bundles communicate with one another and with 

 the axial strand by means of anastomoses at the nodes. 



A B 



FIG. 41. Structure of central cylinder of root in Potamogeton, A, P. natans, L. 

 gef, vessel; s, sieve tube with companion cell; p, pericycle; cj, conjunctive tissue 

 (x 470). B, P. densus, L. Similar to P. natans, but vessels and endodermis thin- 

 walled; sieve tubes shaded (x 470). C, P. pectinatus, L., xylem reduced to single 

 vessel (x 470). [Schenck, H. (1886).] 



We have so far been considering the anatomy of the leafy 

 shoot alone. It should be noted that the structure of the hori- 

 zontal rhizome and of the inflorescence axis are often markedly 

 different. For instance, in the creeping stem of P. pulcher^ the 

 central cylinder takes the form of " a truly dicotyledonous look- 

 ing ring of collateral bundles," while the flowering axis of 

 P. natans also has its vascular strands arranged in a regular 

 ring 2 . 



1 Sanio, C. (1865) first recognised that the apparently simple axial 

 strand of certain Potamogetons was really the reduced representative of a 

 whole system of bundles. 



2 Raunkiaer, C. (1903) and Chrysler, M. A. (1907). 



