48 THE PLANT: A GENERAL EXTERNAL VIEW 



sometimes do work usually done by stems, and stems 

 sometimes do work usually done by roots. In machinery 

 each part has a certain function and can perform no other. 

 But in this matter plants are not like machines at all. 

 Often we find organs doing work 

 which may not be their usual func- 

 tion. Thus in corn you may have 

 noticed roots which act as little 

 props at the bottom of the stem. 

 (See Figure 4.) They are more in 

 the air than they are in the ground. 

 In greenhouses you may have seen 

 those strange plants, the orchids, 

 many of which live without touch- 

 ing the ground. Their roots dangle 

 in the moist air of their tropical 

 homes and absorb moisture directly 

 from it. 



The use of underground parts of 

 plants for reproduction may be al- 

 ready familiar to you. The pro- 

 duction of plants from bulbs is an 

 Fig. 4 . — The prop roots of important part of gardening, onions 

 being an example. Tubers may be 

 cut up and the pieces planted to get a new crop, potatoes 

 being an example. But neither onion bulbs nor potato 

 tubers are roots, as you shall see. A tuber is a swollen 

 part of an underground stem. A bulb is an underground 

 stem and leaves, most of it being leaves. They are men- 

 tioned here to exclude them rather than to include them, 

 to avoid confusion rather than to hold to the subject 

 very strictly. 



--^r 



