ORIGIN OF SPECIES IN NATURE. 



268 



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 Richardia JEthiojnca, 

 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 betiveen 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 heatwards." 

 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, asPlantago maritima, 

 Samphire, &c. This is due to the presence of salt, and it can be 



