100 THE NATURE OF LIFE 



velops into something which superficially 

 resembles a cactus far more than a po- 

 tato. The stem is very short and thick, 

 with nodes close together, the leaves are 

 reduced to small growths resembling 

 spines, and the whole aspect is that of a 

 desert plant. A photograph of such a 

 plant might easily be mistaken for that 

 of a cactus, even by botanists. 



In some aquatic plants the form of the 

 leaf is determined by their position with 

 reference to the water. For example the 

 Mermaid Weed {Proserpinaca) produces 

 under water a leaf which is composed of 

 fine threadlike parts arranged on each 

 side of a central rib. Above the water 

 the same plant produces a flat elongated 

 leaf resembling that of land plants. By 

 growing the plants in an aquarium and 

 alternately lowering and raising the water 

 level the plant can be made to produce 

 each type alternately. 



Light produces remarkable effects upon 

 plants. Not only does it cause the upper 

 and lower sides of the leaf to differ 



