362 BOTANY 



Modifications of Stem. Some of the more striking modifications 

 of the stem in Angiosperms (Figs. 324-326) have been described in 



a former chapter. 

 These include the 

 various subterra- 

 nean forms (Bulb, 

 Corm, Tuber, Rhi- 

 zoine), which are 

 reservoirs of re- 

 serve food ; Run- 

 ners, or Stolons, 

 and similar pros- 

 trate stems, are im- 

 portant organs of 

 propagation; while 

 twining stems and 

 FIG. 326. Stem-tendril of Cucumber. (After BAILEY.) the stem-tendrils 



are adaptations for 



assisting plants to reach the light. The strong thorns of such trees 

 as the Honey-locust (Gleditschia) and the Hawthorn are stem- 

 structures which are presumably protective. 



The development of succulent green stems where the water supply 

 is deficient, is connected with a reduction or complete suppression of 

 leaves, and is obviously to reduce the surface exposed to evaporation. 



The Leaf 



The various forms of foliage leaves, already described in Chap- 

 ter II, are all found among the Angiosperms. The Dicotyledons 

 offer much greater variety in this, as they do in other respects, than 

 do the Monocotyledons. 



Modified Leaves. The leaf, like the stem, may be greatly modified 

 for special functions. Scale-leaves, such as those in scaly bulbs, or 

 winter-buds, consist of the leaf -base only, as may be readily seen in 

 some instances where there are transitions between them and the 

 typical foliage-leaves. 



In submersed aquatics, like Naias or Myriophyllum, the leaves 

 are either linear, or divided into slender divisions, and the epidermal 

 cells are not cuticularized, nor are stomata developed. In xero- 

 phytes, i.e. plants of arid regions, the leaf surface is reduced, and 

 sometimes the leaves are very thick and fleshy, as in species of 

 Agave and Aloe. Parasitic plants, not needing organs for photo- 

 synthesis, have the leaves rudimentary. 



Spines, tendrils, and the traps like those in the Pitcher-plants and 

 Bladder-weed, are also foliar structures. 



