176 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



rule stand erect in air. They need the support 

 of the dense sea- water to buoy them up, aided, in 

 the bladder-wrack, gulf-weed, and others, by the 

 presence of air-bladders or floats. In the atmos- 

 phere these would be quite useless in the way of 

 affording- support, and the surface of the plant could 

 not be fully exposed to the sun's rays. Even were this 

 difficulty overcome — if a sea-weed by some means 

 could acquire rigidity, — yet, being destitute of leaves, 

 in the ordinary sense, it could not draw up water 

 into the higher portions of the frond. A moss, for 

 examj)le, when dry may be moistened at one point 

 without the moisture passing into the other parts. 

 In the higher plants the ascent of the sap depends 

 mainly on the amount of evaporation Tvhich takes 

 place from the surface of the leaves. In a sea-weed 

 there is no such provision ; any internal circulation 

 of fluid that occurs must be due simply to chemical 

 reactions taking place in the cells, and to endosmose. 

 Such circulation cannot be other than sluggish, and 

 quite insufficient for j)lants living in an atmosphere 

 not saturated with moisture. 



By way of contrast, consider how ill adapted for 

 growing on the sea-bottom is an ordinary tree like 

 the oak or elm. Its roots could get but a feeble 

 grasp of tlie rocky sea-bed ; and they would no longer 

 be of use for absorbing water and mineral salts if 

 the whole tree were submerged. The rigidity of the 

 stem and branches would also prove disadvantageous ; 

 and from the great resistance offered to the waves, 

 the branches must in all likelihood be wrenched off^ 

 Even an unbranched stem, like that of the palm, 

 would simi)ly be a lever enabling the currents easily 

 to uproot the tree. The preservation of a sub- 

 merged forest must therefore be quite an excep- 

 tional thing, only x:)ossible where the trees have been 

 rooted in tenacious soil and the waters quiet. Even 

 in such favourable circumstances it must be rare to 

 find an erect trunk of any considerable height. 

 From their fragile nature, the leaves of a submerged 



