SPECIAL FUNCTIONS AND FORMS OF STEMS 87 



cot 



like that of the May apple (fig. 69), the mints, couch grass, and 

 many other plants, and some stouter kinds, like that of the tril- 

 lium and Solomon's-seal, are known as rootstocks, or rhizomes. 

 The very short shoots with disk-like stems and a covering of 

 scales, or of coatings formed by thickened bases of leaves (famil- 

 iar in some lilies, the tulip, and the onion), are called bulbs. 

 Much like bulbs, except that the stem is more developed and that 

 the scales are almost lacking, are tubers, like those of the Jeru- 

 salem artichoke, the potato, and the crocus. 1 The potato is a 

 particularly good tuber for study, as it has well-defined nodes 

 and internodes ; the buds (eyes) are arranged in a distinctly 

 spiral manner and are borne in the axils 

 of little scales which represent leaves, 

 and not infrequently the tuber is con- 

 siderably branched. 



85. Reproduction by portions of the 

 stem. Some plants naturally reproduce 

 themselves mainly by more or less spe- 

 cialized portions of the stem, and in a 

 cultivated state many others are made 

 to do so. There are numerous kinds, 

 such as the potato, the strawberry, the 

 banana, and most lilies, that are almost 

 always propagated by some sort of stem 

 or shoot. 



Many plants bear small aerial bulbs 

 or tubers on some portion of the stem 

 and are commonly reproduced by these. 

 Familiar examples among cultivated 

 plants are the onion and the tiger lily. 



The bulblets known as onion sets are for sale at every seed 

 store, and in some parts of the country are almost exclusively 

 planted by onion growers, while in other sections the seed is 

 more generally planted. The black bulblets of the tiger lily 



FIG. 70. A potato seed- 

 ling ten weeks old 



cot, cotyledons ; st, tuber- 

 l>e;iriug underground stems; 

 t, very small tubers; r, 

 root. Three fourths natu- 

 ral size. After Percival 



1 Such very short underground stems as that of the jack-in-the-pulpit and 

 the crocus are often called corms. 



