938 



^. The Sparganium-Potamogeton association extending from about 

 50Cms. to 100— loOCms. depth^ 



The Litorella association includes in addition to Litorella, 

 Isoetes lacustre and /. echinosporum, Subularia aqnatica, submerged 

 specimens of Juncus supinus, Callitriche hamiilata, and as rarer spe- 

 cies: Lobelia dortmanna, Ranunculus reptans and Potamogeton fili- 

 formis; Nitella opaca is rather common, while Chara delicatula 

 occurs occasionally. 



The first four of the piants named, as well as Lobelia and Ra- 

 nunculus, bear only radical leaves which are short, more or less 

 fleshy, cylindrical and often pointed. Potamogeton filiformis, Calli- 

 triche, the Characeae and as a rule Juncus supinus have well- 

 developed stems, but approach the first type in leaf-form, which 

 is, however, somewhat more slender (linear or setaceous). All grow 

 quite submerged, at any rate as regards vegetative organs, and they 

 are adapted to a submerged habitat in shallow water, hence ex- 

 posed to the influence of the waves. In this respect they are much 

 better adapted than those species with longer shoots and leaves 

 liable to be torn against the stones or gravel of the bottom. 



The different species vary considerably as regards structure of 

 the shoot and vegetative propagation. Subularia and Nitella (and 

 occasionally Callitriche) are annuals, the others are perennials. Lo- 

 belia and the species of Isoetes are sedentary (spot-bound), Litor- 

 ella and Ranunculus reptans have surface runners while Potamoge- 

 ton filiformis has subterrean rhizomes with special winter-buds 

 (hibernacula). It is noteworthy that the phanerogams flower spa- 

 ringly and rarely form perfect seed. I have seen Litorella in bloom 

 only once in a lake on Sydero, and yet it grows in almost every 

 lake. The other species are found in flower more frequently, but 

 only Callitriche and Subularia bear fruit in abundance (compare 

 p. 916). As regards poUination Litorella, Callitriche and Potamoge- 

 ton filiformis are pollinated by wind, but Callitriche, and probably 

 also Subularia, are capable of self-pollination, when the water is 

 high and the flowers submerged, thus accounting for the regularity 

 of fruiting. Ranunculus reptans and in particular Lobelia are pos- 

 sibly insect-pollinated, but may perhaps be capable of self-pollination. 



^ Most of the lakes at greater depths are probably destitute of vegetation, j^et 

 I collected near the shore of Sandsvatn on Sando a large number of small halls 

 of Aegagropila Martensii, which were strewn about on the bottom and doubtless 

 had grown further out in the lake. 



