STiSMS. 



47 



In the simplest form of rootstock, Tig. 33, such as is found 

 in some mints and in many grasses and sedges, the real 

 nature of the creeping stem is shown by the presence upon 

 its surface of many scales which are reduced leaves. In the 

 stouter rootstocks, like that of the iris, Fig. 34, this stem-like 

 character is less evident. The potato is an excellent example 

 of the short and much thickened underground stem known 

 as a tuber. 



It may be seen from Fig. 35 that the potatoes are none of them 

 borne on true roots, A, but only 

 on subterranean branches, e, e, 

 which spring from buds formed 

 in the axils of the cotyledons. 



Fis. 31. — Coiling of Petiole of Dwaxf 

 Tropseolum ; I, leaf ; a, petiole. 



Fig. 32. — Twining Stem of Hop. 



Bulbs, whether coated like those of the onion or scaly like 

 those of the hyacinth, Fig. 36, are merely very short and 

 stout underground stems, covered with closely crowded scales 

 or layers, which represent leaves or the bases of leaves. Fig. 37. 



The variously modified forms of underground stem just 

 discussed, illustrate in a marked way the storage of nourish- 



