158 THE FRESH-WATER LOCHS OF SCOTLAND 
restrial plant is compared with one of aquatic habit, it is found that 
their external and internal morphology differ markedly. On the 
other hand, plants that inhabit very moist environments exhibit an 
intermediate stage. 
From the foregoing remarks it must not be imagined that our 
aquatic flowering plants are the representatives of the ancestral stock 
from which the terrestrial types have arisen. It is rather the 
opposite, for there are good reasons for considering the existing 
aquatic phanerogams to have taken their origin from terrestrial types ; 
not because they were first driven off the land by more robust 
competitors, as is sometimes stated, but, more probably, because 
certain mutable forms have exhibited a tendency, as some do even 
now, to take on the aquatic habit, that mode of living being more 
agreeable to their requirements. Many plants have both aquatic and 
terrestrial forms : when submerged, their structure will exhibit the 
points of typical aquatic types : when growing out of the water, they 
tend towards the morphological structure of terrestrial plants : such 
plants are amphibious. What then is the difference between these 
two opposite forms of plants ? 
The normal terrestrial plant takes in the greater part of its water 
and nutrient salts from the hygroscopic water of the soil by means 
of the osmotic action of delicate hairs and tissue situated near the 
extremity of the rootlets. This sap, beyond providing for the 
maintenance of turgidity, is of little use to the plant until it has 
been conducted into the leaves. There, with the carbon derived 
from the atmosphere, it is converted into various substances for the 
present and future requirements of the plant and its offspring. 
Hence the necessity to such plants for the means of rapidly transport- 
ing this ascending sap from the root- tips to the leaves. The stem 
being aerial, there is no necessity for the storage of a very large 
amount of air. In accordance with the oecological conditions that 
exist where they grow, terrestrial plants have to provide themselves 
with a more or less thickened cuticle in order to prevent the untimely 
evaporation of the sap. 
The physical properties of water induce in aquatic plants con- 
ditions of structure quite different. Water and the nutrient salts 
in solution are absorbed by the whole of the submerged plant-body. 
Instead of being under the obligation to search for food-salts, fresh 
supplies are constantly being brought by the currents, which, owing 
to physical and mechanical causes, are of never-ceasing occurrence. 
Being therefore semi-independent of the root system, the latter has 
usually but feeble development and is often not produced at all. In 
order that this general absorption may take place, and the sap being 
in no danger of undue evaporation, no thick cuticle is developed on 
