112 THE STRUCTURE AND WORK OF THE STEM 



other stems the conditions of life are such that the plant has come to 

 store water in the stem. The cactus, which we shall examine more in 

 detail later, is a plant that has developed the stem for the storage of 

 water, and is so adapted to desert conditions as to prevent the evapora- 

 tion of water from the plant. 



The potato tuber is simply a much thickened -storage stem, as one may 

 easily prove by examination of the so-called "eyes" of a sprouting po- 

 tato. The tiny projection growing within the eye is a bud, which may 



give rise to a branch later. Food and water 

 are stored with the tuber. 



Underground Stems ; the Rootstock. 

 Other stems not only contain stored food, 

 but run underground for the protection of 

 the plant. Such a stem 

 is the rootstock of the iris. 

 Some underground stems 

 do not store food, but 

 grow with considerable 

 rapidity, thus covering 

 ground and starting new 

 outposts of the plant at a 

 distance from the original 

 plants. The pest called 

 quick grass or couch 

 grass, found in almost 

 every lawn, has such a 

 stem. It may be cut in 

 pieces, but each piece 

 may strike root, thus 

 multiplying the plant. 



Bulbs. In the bulb 



of a lily or the onion the stem is covered with thickened 

 leaves, the whole making a compact and reduced plant 

 which, because of its stored food, enables the plant to 

 make an early start in the spring. 



Reduced Stems. In some plants the stem is so re- 

 duced as to be almost lost. This may be of a distinct 

 advantage to the plant in enabling it to escape destruc- 

 tion from enemies. Such a plant is the common dan- 

 delion,, which, because of its short stem, escapes grazing 

 animals and the knives of lawn mowers. Many other 

 low-lying weeds are partly immune from dangers which 

 beset taller plants. 



Climbing Stems. Stems may twist around an object in order to climb. 

 Such a plant is the morning-glory. Here the stimulus which draws tho 



Longitudinal section of a lily 

 bulb. Note the much thick- 

 ened leaves, and the flower 

 cluster at the center. Pho- 

 tographed by Overton. 



Catbrier ; the ten- 

 drils (T) are 

 modified sti- 

 pules (parts of 

 leaves); Th, 

 thorn. 



