CH. v] THE PONDWEEDS 59 



form mats at the bottom of the water, retaining the soil in their 

 meshes and thus consolidating it, while, from these rhizomes, a 

 forest of leafy shoots rises into the water 1 . An examination 

 of the individual axes shows the branch system to be sympo- 

 dial 2 . The shoots are of two kinds; the first is horizontal, more 

 or less buried in the soil, colourless and scale-bearing, while the 

 second is erect, floating to some degree, and producing perfect 

 leaves. Fig. 37, p. 60, illustrates the general scheme of branching. 

 The creeping stem is a sympodium formed by the union, end to 

 end, of the two first internodes of successive generations (I, II, 



FIG. 36. Potamogeton perfoliatus, L. Detached floating shoot, October i, 1910, 



showing how much the plant at this time of year may depart from the perfoliate 



leaf type. Several ''winter shoots" have developed. (^ nat. size.) [A. A.] 



Ill, etc.), the succeeding internodes constituting the erect stem. 

 In one season a great many of these rhizome units may be formed. 

 The first scale leaf of each erect shoot (#, a', a", a'") bears a 

 reserve bud on its axil, which may give rise to another segment 

 of rhizome, again repeating the entire process, so that the whole 

 ramification becomes extremely complicated. In Fig. 37, IF, 

 I IF, represents a reserve shoot, arising in the axil of c, the 

 third scale leaf of Shoot I. By the decay of the older parts of 

 the rhizomes fresh individuals become separated, and even the 



1 Graebner, G. in Kirchner, O. von, Loew, E. and Schroter, C. 

 (1908, etc.). 2 Irmisch, T. (i858 3 ) and Sauvageau, C. (1894). 



