vj STEM ANATOMY OF PONDWEEDS 63 



represents the evolutionary history, since it is possible that 

 certain forms with floating leaves may have had a submerged 

 ancestry. The species whose central cylinder diverges least from 

 a normal terrestrial type, seems to be Potamogeton pulcher^ 

 (Fig. 39 A). Here a section across an internode of the leafy 

 shoot reveals, within the central cylinder, three distinct bun- 

 dles (/! , jTj_ and / x ) which are the traces of the leaf immediately 

 above, and three more (/ 2 , T 2 and / 2 ) which entered at a still 

 higher node. In addition there are several bundles which are 

 purely cauline. The type represented by our native P. natans 

 (Fig. 39 E] differs from that of P. pulcher in the fact that the 

 traces belonging to each leaf do not so fully retain their inde- 

 pendence in the central cylinder. P. perfoliatus belongs to the 

 type of P. natans. In P. crispus (Figs. 39 C and 40 A^ p. 64) the 

 stele is more condensed, the bundles being collected into three 

 groups. In very slender stems of this species, the two passages 

 in each group representing the xylem may fuse so that the 

 distinctness of the bundles is maintained by the phloems alone. 

 P. lucens 2 (Fig. 40 R) has a median and two lateral bundle- 

 groups, but these are more reduced the median group con- 

 sisting of one xylem passage and two phloem regions, and the 

 laterals, of one xylem passage, and one patch of phloem. In 

 this species the tendency to concentric arrangement begins to 

 make itself felt. In P. pusillus (Fig. 40 C) the lateral bundles 

 are entirely fused with the median, as far as the xylem is con- 

 cerned, but the phloems still remain distinct. In P. pectinatus 

 (Fig. 40 Z)) the ultimate term in the reduction series is reached : 

 a ring of phloem surrounds a single xylem passage. Zannichellia 

 closely resembles P. pectinatus \ ephemeral xylem vessels have 

 been detected in the apical region of the stem 3 . In the case of 

 the related genus Althenia^^ vessels are also retained in this 

 region and in the nodes. 



1 Chrysler, M. A. (1907). 



2 On this and other species, Sauvageau, C. (1894) should be consulted. 

 His account diverges in some points from that of Schenck. 



3 Schleiden, M. J. (1837). * Prillieux, E. (1864). 



