The Moisture Relations of Terrestrial Algae . 1 
I. Some General Observations and Experiments. 
J 
BY 
F. E. FRITSCH. 
With two Figures in the Text. 
A. Introductory. 
I T will be a familiar fact that a number of the subaerial algae, common 
in the British Isles, ordinarily occupy habitats which are liable to 
extreme desiccation at certain seasons of the year. Such are : the species 
of Pleurococcus and Trentepohlia found on tree-trunks, wooden palings, &c. ; 
Hormidium flaccidimi and Prasiola crispa , frequent on relatively heavy 
soils ; Zygnema ericetorum and various terrestrial Desmids, characteristically 
found on acid soils ; and, not so widely distributed, diverse Blue-green 
Algae. These forms appear little affected, in nature, even by prolonged 
drought, during which they retain much their normal appearance. On the 
advent of moisture vigorous growth is promptly resumed, and at such 
times the alga may often spread rapidly over adjacent bare areas. Where 
there is little or no competition with other forms, the alga thus gradually 
extends its domain, although often crowded back by other vegetation better 
equipped for absorbing moisture from the substratum (cf. Piercy, 1917, 
p. 515 )- It is to be noted that for all these forms the milder months of 
the winter are essentially the periods for rapid growth and reproduction, 
although in a damp summer such may go on almost continuously with 
occasional breaks during dry weather. 
The striking characteristic of the algae under discussion is the capacity 
of the ordinary vegetative cells, without any marked change and without 
special thickening of the wall, to withstand prolonged drought — a faculty 
otherwise possessed only by the special resting cells (akinetes, aplanospores, 
zygospores, oospores) formed by aquatic and semi-aquatic algae. More- 
over, the change from the resting to the active conditions and vice versa can 
1 From the Botanical Department, East London College, University of London. 
[Annals of Botany, Vol. XXXVI. No. CXLI. January, 1932.J 
B 
