ORIGIN OF SPECIES IN NATURE. 2G8 
other, then all the submerged foliage dies, but new foliage is 
immediately developed suitable to the changed medium. 
A common cultivated example is seen in Bichardia JEthiopica, 
usually grown in pots, but it is an aquatic plant in its native 
habitat. 
There is no apparent reason why water-lilies should not be 
grown in a garden border; if the experiment be made, either by 
sowing the seed or by gradually adapting the thick rhizome to 
put out suitable roots, by supplying it with a wet soil at first. 
The experiment is worth trying, either with water-lilies or any 
other aquatic plants which might be thought suitable for the 
garden. 
As water has the effect of producing degeneration of the 
tissues in aquatic plants as compared with land plants, they 
often grow stronger when on land than when in their normal 
condition under water. 
Changes between an Erect and Prostrate Habit. —The erect 
habit of growth is common with plants growing thickly together, 
but if they are isolated on an exposed surface they will often 
assume a prostrate habit. This is due to the ground being 
warmer than the air above it. We may describe this tendency 
by the term “ thermotropism,” i.e. “ a turning heat wards.” 
This response to an inequality of temperatures will account for 
the plants acquiring a prostrate habit. 
Thus Malva sylvestris, the common Mallow, when growing 
in shady places with other herbs will be erect, but on the road¬ 
sides it becomes perfectly prostrate. It is so prevalent in this 
condition on the limestone of Malta, that it has been named 
M. Nicceensis, but it is simply a prostrate and more hairy form 
of the common Mallow. A very familiar example is seen in the 
lesser Bindweed, Convolvulus arvensis , for this plant is a true 
stem climber when growing among other erect plants, but 
assumes a thoroughly prostrate habit when growing on banks by 
roadsides, &c. 
Similarly a prostrate form is characteristic of high Alpine 
plants, and when lowland plants are grown in those regions they, 
too, then become prostrate in habit. 
Fleshy Types. —Many seaside plants are remarkable for the 
fleshy character of their leaves and stems, as Plantago maritima, 
Samphire, &c. This is due to the presence of salt, and it can be 
