250 



ELEMENTARY STUDIES IN BOTANY 



that any node placed in proper conditions may strike root 

 and put out a branch. Heaping up the soil (" hilling ") 

 about the base of the potato plant induces the formation 

 of more of the subterranean tuber-bearing branches. In 

 the tuber called Jerusalem artichoke, which is developed by 

 the subterranean stems of a kind of sunflower, the nodes 

 of the stem and the buds of branches are more conspicuous 

 than in the potato. Fleshy roots, such as 

 those of the sweet potato, should not be 

 confused with tubers. 



(3) Bulb. In some plants the main 

 stem is very short and is covered by 

 numerous thickened, overlapping leaves 

 or leaf bases (usually called scales), the 

 whole structure being a bulb. Bulbs such 

 as those of the lily, hyacinth, tulip, and 

 onion are very familiar. In this case 

 the food storage is chiefly in the scales. 

 Scaly bulbs are those in which the scales 

 overlap, but are not broad enough to 

 inwrap those within, as the lily bulb (Fig. 

 218) ; coated bulbs are those in which the 

 broad scales completely inwrap those within, as the bulbs 

 of onions and tulips. Small bulbs, called bulblets, are 

 borne by some plants on parts above ground ; as, for ex- 

 ample, the bulblets that appear in the axils of the leaves 

 of the tiger-lily and those that replace flower-buds in the 

 common onion (" onion sets ") These bulblets, when planted, 

 have the power of producing new plants, as do the sub- 

 terranean bulbs. 



These subterranean shoots, with their storage of reserve 

 food, enable plants to put out their aerial parts with re- 

 markable promptness and develop them with great rapidity. 

 As an illustration of a situation in which this ability is of 

 great advantage to plants, the vernal habit may be mentioned. 



FIG. 218. The scaly 

 bulb of a lily. 



