160 PLANTS MODIFIED BY THEIR SURROUNDINGS 



to live under certain conditions are said to be adapted to live under 

 such conditions. Such plants as are best fitted to live under certain 

 conditions are the ones which will survive. 



Water Supply. — Water supply is one of the important factors 

 in causing changes in structure of plants. Plants which live en- 

 tirely in the water, as do many of the algae, have slender parts, 

 stemlike, and j-et serving the place of a leaf. The interior of such 

 a plant is made up of spongy tissues which allow the air dissolved 

 in the water in which they live to reach all parts of the plant. 



If the plant has floating leaves, as 

 in the pond lily, the stomata are 

 all in the upper side of the leaf. 



Plants living in water have not only 

 loose and spongy tissues, but many 

 large intercellular spaces are found in 

 stems or leaves. In one pond lily 

 ( Nelumbo luiea) these spaces in the 

 leaf communicate with large spaces 

 in the veins of the leaf, and these in 

 turn with spaces in the petiole, stem, 

 and root, so that all parts of the 

 plants are in communication with the 

 air above. The roots of a plant living 

 whoUy in water are not needed for 

 support, hence they are often short 

 and stumpy. They do not need to 

 be modified to absorb water ; conse- 

 quently the absorbing surface lacks 

 root hairs. The whole plant, when under water, is usually modified to 

 take water and material used in food-making from its immediate environ- 

 ment. 



Hydrophytes. — If water is present in such quantity as to satu- 

 rate the soil in which the plant lives, the conditions of its environ- 

 ment are said to be hydrophytic, and such plant is said to be a hydro- 

 phyte. 



Xerophytes. — If we examine plants growing in dry or desert 

 conditions, as cactus, sagebrush, aloe, etc., we find that the leaf 

 surface is invariably reduced. Leaves are reduced to spines in 

 the cactus. Some plants, such as the three-angled spurge, which 

 bear leaves in a condition of moderate water supply, take on the 



A water plant, showing the finely 

 divided leaflike parts. 



