MORPHOLOGY OF HIGHER PLANTS. 



327 



not be confounded with tuberous roots, as those of the sweet 

 potato and jalap, for these latter have the morphological char- 

 acters of roots (compare Figs. 185 and 186). 



Instead of the node, or internode, or both, becoming exces- 

 sively thickened, they may be reduced in size and crowded upon 



tl- 



FIG. 185. A potato plant grown from seed and showing the branches upon which the 

 potato tubers are formed, r, primary root; ct, cotyledons; c, hypocotyl; f, foliage leaves; 

 f', a primary branch the summit of which has developed foliage leaves; e'c, scales on upper 

 portion of primary branch; e'c', scales representing the eyes of the potato tubers formed 

 from the swollen branches; br, buds formed in the axils of the scales on the tubers; r', sec- 

 ondary roots formed on the stem branches. After Duchartre. 



each other, the leaves at the same time becoming thickened and 

 filled with nutriment. Such a modified stem and leaves, as in the 

 onion and tulip, is called a BULB (Fig. 188). Bulbs are sometimes 

 produced in the axils of the leaves of overground stems, as in some 

 lilies, and are then called bulbils or bulblets. They are also found 

 in Allium forming what are commonly known as " onion sets." 



