460 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



of 9' above 0, being often fairly broad, but never very 

 high. It is a plant which is completely exposed only for 

 a short timj 



Little is known as to the reason why longer and shorter 

 exposure causes a difference in the habit of an alga. We 

 practically know nothing about the distribution of and 

 the meaning of the plant forms met with in algse. 



Long submergence in sea water is evidently conducive 

 to increase in size and strength. This is possibly due to 

 the necessity of providing for an increase in assimilating 

 power. The forms which are left exposed long become 

 smaller and often rather close set. Plants with bladders 

 are restricted to a limited area, which is probably exposed 

 at every tide, but the significance of the bladders, from 

 an ecological point of view, I have not yet been able to 

 fathom. Halidrys siliquosa occurs, with bladders, quite 

 isolated, at great depths. 



So far it can only be said that marine algse, as a whole, 

 are at their best when least exposed. Certain species, 

 however, by the possession of certain structural or other 

 peculiarities are able to live in localities which must be 

 considered less favourable. They were driven there by the 

 strong competition prevailing in better localities. Pelvetia 

 canaliculata was probably unable to stand the competition 

 of the moister parts of the sea shore, and was thereby 

 driven to its present position. Many of the green algse 

 seem to be at their best in the higher regions. A large 

 amount of light seems to be necessary for their well being. 

 Many ChlorophyceaB seem to be quite in-different to 

 changes in the salinity of the sea water caused by an 

 inflow of fresh water. 



The point of greatest interest is still to ascertain that 

 factor, the influence of which the plants have to guard 

 against during exposure. Is it the strong light, or is it 



