302 PHYSIOLOGY, SYMBIOSIS, ETC. 



course of their cultivating operations must frequently be respon- 

 sible for the conveyance of algae down into the soil. 



Many of the soil algae, especially the Cyanophyceae, 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 

 Cyanophyceae appeared last, nevertheless they quickly became 

 dominant. Nostoc muscomm and Nodularia Harveyana appeared 

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

 inianum and Anabaena 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. 192) 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 protoplast. 



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

 tendency for the cells to plasmolyse and there are also changes in 

 the permeability of the cell wall, whilst the access of moisture 

 normally brings about changes in the reverse direction. The 

 majority of cells which do plasmolyse lack the characteristic 

 granules, mainly of fat, that are to be found in most terrestrial 



algae. 



(5) Those cells which survive after drought do not contain any 

 vacuoles and possess instead a rigid, highly viscous protoplast which 

 is in a gel condition. This is the normal state of the vegetative cells 

 of Pleurococcus and the cells in the '' Hormidium'' stage of Prasiola. 



(6) If desiccation is rapid most of the cells will die but some will 

 plasmolyse and retain their vitality in that state for weeks or months. 

 In spite of the death of the bulk of the cells no species disappears 

 from the flora during a rapid onset of drought. 



(7) If desiccation continues, the number of living resting cells 

 will remain constant for several years. 



