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A TEXTBOOK OF BOTANY [Ch. IV, 11 



where the nearly globular storage stem is commonly mis- 

 taken for, and called, a bulb (page 73). All of these 

 stems produce roots, and also give rise to the foliage; 

 but cases occur in which food-storage completely displaces 



the foliage-supporting function, 

 and also the production of roots. 

 Then we have a new organ, ex- 

 emplified in the common potato, 

 the stem nature of which is 

 attested by the eyes, which are 

 axillary buds subtended by small 

 scale leaves. Such an organ, 

 rotund with accumulated food, 

 and composed mostly of thin- 

 walled rounded storage cells of 

 the greatly developed pith and 

 cortex, is called a tuber, of which 

 many forms occur among plants. 

 Another important special func- 

 tion of stems is represented in 

 tendrils, which have the same 

 elongated slender forms, move- 

 ments through the air, thigmo- 

 tropic twining about a support, 

 and spiral shortening, already 

 described in leaf tendrils (page 

 77). Passion Vine, Wild Cu- 

 cumber, and Grape Vines have 

 stem tendrils (Fig. 136), which are more abundant and 

 perfect in form than leaf tendrils, perhaps because support 

 is a more natural function of stems than of leaves. 



Stems also become transformed into spines, which are 

 sometimes very large, as in Honey Locust (Fig. 137). The 



Fig. 136. — Tendrils, from 

 axillary buds, in a Mexican 

 Passiflora. 



Compare also Fig. 52. The 

 tendrils of Grape Vine and all 

 of the Gourd family (Squash, 

 Wild Cucumber), represent the 

 main stem, the further growth 

 taking place from the axillary 

 bud. (After Gray.) 





