FRESH WATER ECOLOGY; SOIL ALGAE 379 



through their own motiHty, further experimental work on this 

 aspect is much to be desired. The effect of water seepage will de- 

 pend on the heaviness of the rainfall, the state of the soil, i.e. 

 whether dry and cracked, and the nature of the algae, i.e. whether 

 or not they possess a mucous sheath. Passage through the soil is 

 facilitated in the filamentous algae either by fragmentation or else 

 by the formation of zoospores, the factors that are responsible for 

 the former process appearing to differ for the various species. Many 

 green algae are known to form zoospores when put into water after 

 a period of dryness, and hence one may presume that a shower of 

 rain will also induce zoospore formation. Petersen (1935) has 

 demonstrated experimentally that rain can carry algae down effi- 

 ciently to a depth of 20 cm., but that the process is facilitated by 

 the presence of earthworms, although these animals probably only 

 operate indirectly in that they loosen the earth. Farmers in the 

 course of their cultivating operations must frequently be respon- 

 sible for the conveyance of algae down into the soil. 



Many of the soil algae, expecially the Myxophyceae, can resist 

 very protracted spells of dryness as Roach (1920) demonstrated 

 when soils from Rothamsted that had been kept for many years 

 were remoistened. Bacteria developed first, then unicellular green 

 algae with some occasional moss protonemata, and although the 

 Myxophyceae appeared last, nevertheless they quickly became 

 dominant. Nostoc muscorum and Nodularia harveyana appeared 

 after the soil had been dried up for 79 years, whilst Nostoc passer- 

 inianum and Anahaena oscillarioides var. terrestris appeared after 

 59 years of dryness. These algae differed in some respects from the 

 typical forms that are to be found in ordinary soils, but this was 

 probably only due to the cultural conditions. 



Fritsch and Haines (1922) have studied the moisture relations of 

 some terrestrial algae (cf. Fig. 205) and they have shown that: 



(i) There is a complete absence or paucity of large vacuoles. 



(2) In an open dry atmosphere nearly all the sap is retained. 



(3) When the filaments dry up, contraction of the cell is such 

 that the cell wall either remains completely investing the 

 protoplast or else in partial contact with it, thus ensuring 

 that all the moisture which is imbibed will reach the proto- 

 plast. 



(4) During a drought there is, as time goes on, a decreasing 

 tendency for the cells to plasmolyse and there are also changes 



