8i6 ZONES AND REGIONS [Pt. Ill, Sect. V 



existing or geological causes account for these differences. On the other 

 hand, widely distributed organisms are not wanting in limnetic plankton. 

 Thus, Asterionella formosa, A. gracillima (Fig. 500, c), Fragilaria crotonensis 

 (Fig. 500, a) (all belonging to the Diatomaceae), Clathrocystis aeruginosa 

 (Cyanophyceae), and Ceratium hirundinella (Peridinieae), are found in most 

 lakes in Europe and North America, to a certain extent also in the Himalayas, 

 and probably they have a still wider distribution. 



As in pelagic, so also in limnetic plankton, the Bacillariaceae pre- 

 dominate. They exhibit adaptations to the floating mode of life which are 

 similar to those of their marine allies, yet do not possess contrivances so 

 complete as do marine species such as Gossleriella tropica or Planktoniella 

 Sol. The adaptations to plankton-life have not advanced so far. The 

 next important class is the Cyanophyceae, which, as in the sea, owe their 

 floating power to small air-bubbles. Other groups of Algae are repre- 

 sented by merely a few species, which are usually less prominent or of local 

 occurrence. Fairly widespread is Botryococcus Braunii (Tetrasporaceae), 

 which is very abundant in the plankton of Lake Constance, and produces 

 hollow spherical coenobia, whose power of floating is caused by air-bubbles 

 in the central cavity that is traversed by threads (Fig. 500, c-g). Of less 

 importance are some Desmidiaceae (Staurastrum), Volvocaceae (Volvox, 

 Eudorina, Pandorina), Protococcaceae (Pediastrum), and Peridinieae (Cera- 

 tium), and others. 



The limit between the photic and dysphotic regions, as regards the 

 limnetic plankton, varies in depth according to the less or greater turbidity 

 induced by particles in suspension. In Lake Zurich, according to 

 Heuscher 1 , plankton is pretty uniformly distributed down to a depth of 

 10 meters, and decreases slowly down to 25-30 meters, but more rapidly 

 at greater depths. From 25-30 meters may therefore be reckoned as the 

 lower limit of the photic region. The various Algae exhibit a distribution 

 in layers. The Cyanophyceae descend only a few centimeters from the 

 surface, so that, where they occur abundantly, as in the Holstein lakes, 

 there is a specially dense superficial plankton. This is also true of 

 Botryococcus Braunii in Lake Constance. On the contrary, the Diatomaceae 

 occur at a considerable depth, without immediately showing any diminution 

 in numbers, and are still abundant at the greatest depths investigated, 

 56 meters in Lake Constance and 90 meters in Lake Zurich. 



3. FLOWING WATER. 



Flowing water differs from still water so much the more as regards the 

 conditions of vegetation, the stronger its streaming movement is. For 

 instance, I saw parts of St. John's river in Florida covered by floating 



1 Heuscher, op. cit. 



