66 POTAMOGETONACEAE [CH. 



A similar reduction series to that met with in the central 

 cylinder of the stem can be traced in the root 1 . In Potamogeton 

 natans (Fig. 41 A^ p. 65) the root is pentarch and the walls of all 

 the elements, except the sieve tubes, are thickened. P. densus 

 (Fig. 41 B) has the same type of structure, but the cell- walls 

 remain thin. In P. fectinatus (Fig. 41 C) the five protoxylem 

 elements are absent, and the xylem is represented merely by 

 a single central vessel with delicate, spiral thickening 2 . The 

 structure of the root of Zannichellia is similar, but the axial 

 vessel is unthickened. 



The Potamogetons tide over the winter in various ways. In 

 P. fectinatus^ the Fennel Pondweed, common in fresh and 

 brackish waters, the leafy shoots give rise to tubers in the 

 autumn. These tubers are usually formed by the swelling of 

 the two basal internodes of that part of the axis which would 

 otherwise become erect and leafy. Each tuber is enclosed in a 

 scale leaf and terminates in a bud ; it contains starch and, as it 

 is easily detached, it forms a means of vegetative multiplication. 



Other species are reproduced by special buds, or turions 3 , in 

 which the leaves, rather than the axis, play the chief part. A 

 group of submerged Pondweeds with linear leaves, of which 

 P. pusillus and P. trichoides are examples, is characterised by 

 winter-buds enclosed in scales corresponding morphologically 

 to axillary stipules accompanied by rudimentary laminae. In 

 this group of species there is no rhizome, branching sympodially 

 in the mud, the only part corresponding to such a rhizome being 

 the elongated axis of the turion; the branched leafy shoots play 

 the chief role in the axial development. The whole vegetative 

 body in these species dies off in the autumn and the turions 

 alone remain. These buds are formed in great numbers, and 



iSchenck, H. (1886). 



2 Sauvageau,C. (18892) describes the roots ofP.pectinatus as having, in 

 general, a less degraded type of structure than that attributed to them by 

 Schenck, H. (1886). 



3 Gliick, H. (1906) deals comprehensively with the turions of the 

 genus. 



