THE ENVIRONMENT OF PLANTS. 
171 
As a good example of one of Nature’s experiments, let us take 
the water Crowfoot ( Ranunculus aquatilis). This plant grows in 
ponds and rivers, &c. In still water it produces very finely 
divided leaves with almost thread-like divisions, all of which 
are always entirely submerged ; but when the stem grows to 
near the surface of the water, it develops ordinary leaves, heart- 
shaped or rounded with a scolloped edge. These float on the 
surface. 
When we examine the tissues of these leaves micro- 
scopically, there are the following differences : in the floating 
leaves there is a superficial cellular skin or epidermis, which 
is composed of a layer of colourless cells like flat boxes, with a 
wavy contour all fitting tightly together like the pieces of a map- 
puzzle. At intervals there are an abundance of “ breathing- 
holes,” or stomata. They are composed of two oblong cells, 
rounded at the ends and curved towards each other, leaving a 
space between them for the circulation of air within and without 
the leaf. The stomata are on the upper epidermis exposed to 
the air. There are none, or at least very rarely one or two, here 
and there on the lower epidermis in contact with the water. The 
intermediate tissue contains the green granules called chlorophyll 
grains. 
In the submerged leaves there is no true epidermis ; the 
outermost layer of cells, which represents it, is full of green 
chlorophyll, and there are no stomata at all. 
There are, indeed, several other differences which need not 
be specified. The conclusion is that the one kind of leaf is 
adapted to water, the other to float on the surface and to have 
at least one epidermis adapted to air. 
Now suppose the pond to dry up gradually during a hot 
summer, so that the water all goes, leaving nothing but mud, 
which is in the course of drying up too. What happens ? Our 
Ranunculus becomes a terrestrial plant. It produces an abund- 
ance of “ dissected ” leaves, very similar in appearance to the 
submerged ones, but not at all identical in structure. The little 
thread-like divisions now grow stouter and can stand erect in 
air. They are a little flattened instead of being circular in a 
cross section. They develop plenty of stomata, and possess a 
true epidermis, which, as is usual with aerial leaves, has no 
chlorophyll except in the cells of the stomata. 
Here, then, we recognise two facts. One, that heredity 
compels the plant to produce leaves like the submerged ones, 
but the new environment compels it to construct the leaves for 
an aerial existence. Sometimes only this sort of leaf is pro- 
duced, and the Ranunculus then carpets the ground with a kind 
of soft green pile. Sometimes it produces a certain number of 
dissected leaves, and then suddenly changes and bears the 
form of leaf which floats. Its habit was too strong to be lost. 
As these leaves are already partially adapted to air, they do not 
alter, their structure being just the same as when floating. 
