Chap. Ill] FRESH-WATER VEGETATION 813 



(Fig. 498). Associated with them are other plants with floating leaves and 

 those with the ends of their shoots projecting above water (Nymphaea- 

 Hippuris type). A condition vital to most of these plants is that a part 

 of their organs shall be in the air. Only a few can live completely 

 submerged (Batrachium aquatile). To the upper belt also belong the 

 plants of the Isoetes type. Some of them (Isoetes) lengthen their 

 leaves as the depth increases, but not sufficiently to obtain any essential 

 advantage. 



To the third belt belong the flowing phanerogams of the Naias type that 

 vegetate quite submerged, but which contrast with those of the Isoetes 

 type by prolonging their axes towards the light. Here the dominant 

 phanerogams are species of Potamogeton, and, at greater depths, species of 

 Naias. Below a depth of 6 meters phanerogams occur in very small 

 numbers. The free, floating and 

 swimming, forms of hemiplankton 

 (Hydrocharis type), as they periodi- 

 cally belong to the vegetation at the 

 bottom (hibernation, seeds) of the 

 water, occur only in the outermost 

 belts. 



From a depth of 2 meters on- 

 wards species of Chara usually 

 form an important part of the vege- 

 tation; with increasing depth species 

 of Nitella gradually appear. At a 

 depth of 7 meters and below this, 



macrophytic vegetation is almost 



... , r at- 11 Fig. 499- Furrowed stone from Langenargen on 



exclusively composed Of Mltella Lake Constance. Half natural size. After Kirchner. 



syncarpa. In its company a few 



individual Musci, such as Fontinalis antipyretica and Hypnum giganteum, 

 appear. The submerged meadows of Nitella continue occasionally in 

 clear water (Lake Constance) to a depth of 30 meters, where their 

 cessation denotes the inferior limit of the photic region. In the more 

 turbid waters of Lake Miiggel in Bavaria, the Nitella-vegetation ceases at 

 a depth of 12 meters, and is replaced down to a depth of 30 meters by 

 sheets of Cladophora, in which Cladophora profunda, Brand, C. cornuta, 

 Brand, and Rhizoclonium profundum, Brand, predominate 1 . Microphytes 

 in great numbers are associated with the macrophytic benthos of the photic 

 region. Bacillariaceae, in particular, form brown flaked coatings on 

 submerged parts of plants, stones, and other bodies. Of greater interest 

 are several lime-excreting Cyanophyceae, which form soft coatings on stones 

 in very shallow calm places in some lakes, and are connected in some 



1 See Brand, op. cit. 



