820 ZONES AND REGIONS [Pt. Ill, Sect. V 



the intercellular spaces, for example Lemna and Ceratophyllum ; still others 

 perennate only by means of their rhizomes, as the Nymphaeaceae and Po- 

 tamogeton natans ; others again only by special winter-buds, like species of 

 Potamogeton, Utricularia, and Hydrocharis. Thus, on the whole, there is 

 on the part of the benthos-plants a descent towards the bottom of the water, 

 where the temperature, except in quite shallow water, is higher than at the 

 surface 1 . 



Limnetic plankton is present throughout the year, although its com- 

 position varies according to the season 2 . During winter Diatomaceae 

 predominate ; some other Algae and Peridinieae are associated with them. 

 On the other hand, a number of forms that occur during the warm season — 

 for instance Ceratium hirundinella, most of the Cyanophyceae and Chloro- 

 phyceae — are absent. Such forms at the commencement of winter produce 

 resting-spores, which sink to the bottom. Some species assume different 

 shapes, according to the season. According to Schroter, the numerical 

 maxima for the majority of species occur in May and August. 



Of the plankton of the lakes in Holstein Apstein 3 gives an account from which the 

 passages relating to the plants are here quoted : — 



The months of January and February are devoted to rest. ' Many Algae entirely 

 disappear from the plankton ; either, like Gloeotrichia, they have formed spores, or, 

 like Ceratium, cysts, all of which sink to the bottom of the lake. Others have 

 become very scarce, but may always be found, such as Chroococcaceae, Pediastrum, 

 and above all the Diatomaceae. 



' But besides the above, the Dinobrya have entirely disappeared, although some 

 isolated plants were still to be found in December; they have already, in July and 

 August, formed cysts that sink. 



'As soon as the sun begins to become more powerful in action and when the 

 deep water becomes warm (April), a great change occurs. Most of the Diatomaceae 

 appear in multitudes, so that for a short time they quite dominate the plankton. 

 Among other plants Dinobrya follow during the spring, and at its termination have 

 reached their maximum ; then again Gymnodinium fuscum appears. All the other 

 plants may then be found, but are at the first stage of their development. . . . 



' During summer, life has attained its highest point, especially as regards the 

 greatest variety of species. The lower Algae, such as Nostocaceae, Rivulariaceae, 

 and Chroococcaceae, attain their highest development and impart to the water colours 

 that are visible to the naked eye. Palmellaceae and Volvocineae are numerous, as 

 are also the Peridinieae, and many of the Diatomaceae attain their maximum at this 

 time or at the beginning of autumn. . . . 



' In autumn many Diatomaceae attain again a more or less considerable develop- 

 ment, but the other vegetable organisms have commenced to undergo a decrease, 

 which takes place very rapidly with some of them, in many cases being accompanied 

 by the formation of resting stages.' 



1 Schenck, I, p. 81. * Schroter, op. cit. ; Apstein, op. cit. 



3 Apstein, op. cit., p. 127. 



