FLORA OF SCOTTISH LAKES 
159 
the submerged portion. This explains why aquatic plants wilt so 
rapidly when exposed to the air. The epidermal cells of aquatic 
plants often contain chloroplasts, which is not the case in terrestrial 
plants. Absorption not being restricted to the tips of distant roots, 
a less elaborate sap-transporting system will suffice. Owing to the 
support afforded by water, the lignified elements of the vascular 
bundles are often reduced to a minimum ; the tissues are mostly 
parenchymatous and the walls of the cells are thin. Being shut off 
from an adequate supply of air, aquatic plants have to arrange for 
the storage of considerable quantities of this necessity ; they have 
therefore provided themselves with very large reservoirs in the form 
of intercellular spaces. These air-spaces are often very large and con- 
tinuous ; a person may, for example, quite easily blow through the 
petiole of a water-lily six or seven feet long. The air in these inter- 
cellular spaces varies with the metabolic activities, and is often held 
under a negative pressure. 
Terrestrial plants, being obliged to protect themselves against 
the excessive evaporation of their sap by means of a waterproof 
cuticle, which is also impervious to air, are under the necessity to 
provide a means by which an interchange of the external air with 
that of the intercellular spaces may be brought about ; at the same 
time this interchange must be under the complete control of the 
individual. All this is accomplished by means of stomata and lenticels. 
The latter are quite unknown in aquatic plants as there is no second- 
ary thickening, and the stomata only occur upon organs exposed to 
the air, as upon the upper surface of floating leaves. Aquatic plants 
often exhibit a marked variance between their submerged and floating 
leaves. Ranunculus peltatus, Potaraogeton heterophyllus, and Apium 
inundatum may be cited as examples, all of which may have broad 
floating leaves and narrow, thread-like submerged ones. Terrestrial 
plants, however, may exhibit similar features in their upper and lower 
leaves, but for a different reason. 
The flora of rapidly flowing water is limited to such species as 
root firmlv in the substratum. Such water contains a much e^reater 
supply of air than does still water ; consequently plants that thrive 
in this kind of habitat have relatively smaller internal air reservoirs 
than have those plants that inhabit still ponds and lakes ; they also 
have a greater development of mechanical tissue. In rapidly moving 
water, plants have a much greater supply of food-salts presented to 
them in the form of matter in solution than is the case in still water 
of the same composition. This, in conjunction with the pull exercised 
upon them by the current, has a tendency to induce the production 
of a larger plant-body, their stems and leaves being elongated in the 
direction of the stream. As a result of this excessive vegetative 
