5 oo 



ECOLOGY 



phenomena are seen along ditches and irrigation canals and on the sides 

 of vertical cliffs. Prohydrotropic root reactions are of great advantage in 

 view of the dominant role of water in the life of plants. Various factors 

 other than water prevent the expression of progeotropic tendencies. In 

 rocky regions, where the strata are horizontal, the roots are unable to 

 penetrate downward. Perhaps the most notable exception to the usual 

 progeotropic reactions are seen in swamp roots (p. 507). 



Food accumulation in roots. While stems are the most conspicuous 

 food-accumulating organs, fleshy roots also are of great importance 

 in this regard, familiar examples being afforded 

 by the parsnip, carrot, turnip, and beet (fig. 711; 

 also fig. 720). Some roots, such as those of the 

 beet, accumulate sugar, while others accumulate 

 starch (as in many orchids). The roots of many 

 Compositae accumulate inulin (p. 914). Water 

 is accumulated in fleshy roots to an even greater 

 degree than are starch and other foods. In arid 

 regions large fleshy roots are frequent, and some- 

 times plants whose stems are small and delicate 

 have roots of enormous size. Various species of 

 Ipomoea have large roots rich in foods; among 

 these is the sweet potato (/. Batatas). The 

 man-of-the-earth (7. pandurata) has much larger 

 roots, and I. leptophylla of the plains has roots 

 that are even gigantic when compared with the 

 relatively small aerial organs. 



There appear to be several advantages associated 

 with food accumulation in roots. Such organs 

 seem better protected from animals and from low 

 temperatures than are aerial organs, and unques- 

 tionably they are better protected from desicca- 

 tion. It seems more than a coincidence that the greatest development 

 of large fleshy roots is in those regions where the danger from drought 

 is the greatest. While the most striking illustrations of fleshy roots are 

 found in arid regions, many plants of our woods and fields, such as 

 the dandelion, dock, evening primrose, and various orchids, have similar 

 roots that accumulate much food and water. 



FIG. 711. A beet 

 (Beta vulgaris), illus- 

 trating a much en- 

 larged primary root; 

 in such roots large 

 quantities of water and 

 food accumulate. 



See p. 911 for a general consideration of the significance of food accumulation. 

 It may be noted here that part of the starch that accumulates in growing rocts seems 



