226 NOTES ON PONDWEEDS. 



occupied space on the surface of the water, the leaves become 

 crisped and undulated to some extent ; but further observations 

 are needed to decide the question of state or variety. Iu relation 

 to this question it may be explained that Pondweeds have two 

 usual modes of defence against competing vegetation : one is by 

 producing narrow grass-like leaves which can exist in a crowd, the 

 other by developing broad coriaceous floating leaves, which, involute 

 in vernation, are enabled to pierce through the dense surface 

 growths, and thfn, expanding, are able to smother such plants as 

 are incapable of rising above the surface of the water. In the first 

 mode they imitate P. pectinotus, in the second P. natans, the two 

 extremes of tbe genus in most arrangements. In another respect, 

 too, P. crispus has some resemblance (or perhaps some reversion) 

 to the pectinatus-growp, that is, hi early growths, made from 

 November to January ; many of the lower leaves are adnate, being 

 sessile on the backs of the stipules. These adnate leaves are flat 

 and narrow, but in other respects resemble the ordinary leaves of 

 the species, and are of common though not universal occurrence ; 

 as many as two to six are to be met with on the main stem, and 

 from one to three on the branches. Like the stipules they are 

 very fugitive, and therefore cannot be found on the more mature 

 growths. Although most frequent in the winter, they are occa- 

 sionally produced throughout the year, especially on the young 

 branchlets springing from the axils of the lower leaves. These 

 adnate lower leaves have already been noticed in P. Friesii, P. 

 perfoliatus, P. pralongus, P. Griffithii, P. decipiens, P. Zizii, P. 

 coriaceus, P. heterophyllus, P. nitem, P. riifescens, P. plantagineus, 

 and P. flwitans. Probably they will be found to occur in all tbe 

 other British species ; they should be looked for when the plants 

 are growing under adverse conditions ; forms which are probable 

 hybrids seem especially liable to produce them. 



I would also call attention to the remarkable fact of the fruit- 

 spikes being borne above the surface of the water. As a rule, the 

 fruits of Potamogeton seem to be matured under water, in some 

 species even after the decay of the stems in autumn ; full observa- 

 tions on this point are much needed, and require to be made year 

 after year, as there are some indications that the season and 

 manner of ripening the fruit is not constant in the same species.* 

 Chamisso and Schlechtendal (' Linnea,' v. 2, p. 187) remark that 

 the fruits are often abortive ; this species, however, possesses a 

 peculiar way of propagation by means of thyrsiform winter-buds, 

 which form in the axils of the leaves, and which are produced in 

 such numbers as to render the plant able to diffuse itself through a 

 large sheet of water in less time than any other species can do by 

 seed. These winter-buds are horny in texture, and have probably 

 greatly aided the dispersion of P. crispus over the greater part of 

 the world. 



There is no species of Potamogeton which will better reward 

 careful investigation ; easily recognised by the beginner, there is 



* Probably the fruit of P. crispus is matured under water, but it seems to be 

 less deeply submerged than that of most species, and liable to frequent exposure 

 to the air, a condition which the thick corky drupelets can sustain without injury. 



