[Chap. XXVI FORMS AND EXTERNAL FEATURES OF STEMS 257 



Fig. 91. Rhizome, roots, and base of an aerial stem of Solomon's seal collected 

 early in July. An erect aerial shoot develops each year from the growing apex ot 

 the rhizome. The round scar formed at the end of the season when the aerial 

 stem abscises is the so-called seal. Following the growth of the aerial shoot a new 

 rhizome segment develops annually. The older segments at the opposite end of 

 the rhizome gradually die and decay. 



Tubers, corms, and bulbs. An underground stem familiar to everyone 

 is that of the Irish potato. The slender rhizomes of the potato enlarge 

 terminally into thickened structures known as tubers. Each eye of the 

 tuber consists of small scales and a cluster of rudimentary buds. When 

 the tuber is planted, an aerial branch develops from a bud in one or more 

 of the eyes, depending upon the degree of apical dominance. Large 

 quantities of starch accumulate in the potato tuber, and it has become an 

 important source of food, even replacing bread made from cereals in 

 some countries. Tuber-like thickening may occur also in aerial stems as 

 in kohlrabi. Some of the so-called short thick roots such as globe radish 

 and turnip, are mostly enlarged hypocotyls. 



Another distinctive type of stem resembling the tuber is the corm, 

 characteristic of jack-in-tlie-pulpit, dasheen, and gladiolus (Fig. 92). 

 It is a short, upright, thick stem commonly covered by thin membranous 

 scale-like leaves. It bears both lateral and terminal buds. 



A bulb consists of a short, upright stem bearing thick leaf bases, 

 axillary buds, and a prominent terminal bud. The underground bulbs of 

 hyacinth, tulip, onion, and garlic are familiar to all. A few bulbs are 

 aerial, such as the sets produced on the flower stalks of onion and 



