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BOTANY 



by experiments, external forces act on the organs of a plant so as to change 

 its appearance and often its form and habit. A stem grown in complete 

 darkness is white instead of green. The bleaching of the celery stems by 

 covering them is a familiar example of this. Thus, in nature, forces which we 

 know of as light, gravity, heat, moisture, wind, and many other factors, 

 influence the plant in its growth. Let us now examine some of the examples 

 of modified stems. 



Stems Modified for Water or Food Storage. — Many stems store 

 large quantities of food. The sago palm is an example of such a stem. In 

 most woody stems food is stored during some parts of the year and is used 

 as the plant comes to need it. In 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 evaporation of water from the plant. 



Underground Stems; the Rootstock. — Other stems not only con- 

 tain stored food but run underground for the protection of the plant. Such 



a stem is the rootstock 

 of the iris. The root- 

 stock in many respects 

 resembles a root, but can 

 be distinguished from 

 this part of the plant be- 

 cause the leaves come 

 out from definite points 

 or nodes and because 

 true roots leave the under 

 surface. Some under- 

 ground 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. 



The Tuber. — If the underground stem becomes thickened at its end 

 and there forms an enlargement for the storage of food, we call such an 

 enlargement a tuber. Its use to the plant is evident when a potato is 

 planted. 



A rootstock, an underground stem. Note the leaf scars 

 CL.6'.), the roots {R.), and the leaf stalk (P.). 



