TO THEIR ENVIRONMENT. 63 



In the latter, they are produced by the drying-up of masses of 

 tissue, as is shown by ragged edges of cell-walls remaining after 

 the drying-up process ; and in the former, by the clean separation 

 of the walls dividing cells, such fringes being absent. These air- 

 spaces, which, when the plant is in a healthy state, are filled with a 

 mixture of gases, practically the same as atmospheric air, and from 

 which it may obtain supplies of carbonic acid, buoy up aquatic 

 plants in the water, and effectually secure mechanical support to 

 them, so that, after all, the non-lignification of cells is really an 

 advantage to such organisms, owing to the greater lightness thus 

 obtained by the reduction of the strengthening strands. This 

 altered structural formation is more clearly shown in those plants 

 which are totally submerged. For example, Anacharis, Hottonia 

 palustris, and Uiriadaria are much more thoroughly aquatic types 

 than Hippuris, Nuphar luteum^ and ATyniphc^a alba. French 

 savants have experimented with normally terrestrial plants, grow- 

 ing them in water, and in doing so have educed another proof of 

 the great influence of environment on plant tissues. Under these 

 circumstances, any new growths were found to assume the aquatic 

 type of structure. 



II. ^-A plant obtains its chief food, carbon, from a gas, carbon 

 dioxide (a compound of carbon) and oxygen, which is present in 

 the air, being provided with little mouths or stomata, through 

 which the gas has access to the internal tissues, where the chloro- 

 phyll or green colouring matter of the cells, in the presence of 

 water, decomposes it, fixing the carbon and restoring the oxygen 

 to the atmosphere. Many of the vital operations in the plant's 

 economy — such as the decomposition of the carbon dioxide and 

 the expulsion of the oxygen already mentioned, the formation of 

 chlorophyll, and the transmutation of the crude sap and the 

 carbon into food material for the plant — can only be carried on 

 under the influence of light. 



It is not, therefore, difficult to comprehend that a plant or a 

 portion of a plant, growing immersed, must undergo modifications 

 of structure to enable it, under the changed conditions of its life, 

 to get its due share of light and air. The submerged leaves, 

 accordingly, become much divided, in order to allow a ready 

 passage to rays of light, and to enable the plant to extend its 



