744 



ECOLOGY 



lessen the intensity of the incident light, thus, perhaps, reducing trans- 

 piration. Thorns are of undoubted advantage in the climbing of many 

 lianas (p. 654). Probably many and perhaps most spines subserve no 

 role of importance; in Gleditsia they occur chiefly on old and inedible 

 branches, while the tender young shoots commonly are spineless; the 

 increased spinescence of xerophytic forms (as in Ulex and Rosa) 

 appears to have no advantage. 



Tuberization. Early experiments. Tubers represent the cul- 

 mination of stem shortening accompanied by diametral enlargement 

 (figs. 1069, 983, 989, 990), and commonly they accumulate quantities 

 of food and water which are utilized when their buds develop into shoots. 

 Experiments show that tuberization is directly favored by darkness and 



FIG. 1069. A rhizome of Scutellaria parvula, in which there has taken place alter- 

 nately stem elongation and tuberization; r, adventitious roots; t, tuberized portions. 



checked by light, although indirectly light favors tuberization in that 

 it stimulates food formation in the foliage leaves. The removal of all 

 rhizomes from a developing potato plant results in tuber formation on 

 aerial shoots, regardless of illumination. Moderate desiccation favors 

 tuberization, while moisture often inhibits tuber formation (as in the 

 potato and in species of Juncus). Low temperatures appear to favor 

 tuberization, only tuber-bearing shoots developing in the Marjolin potato 

 below 7 C., while only leafy shoots develop above 20 C.; horizontal 

 tuber-bearing shoots may be transformed into erect leafy shoots by rais- 

 ing the temperature, and the reverse transformation may be effected by 

 lowering the temperature. More heat is necessary to transform dex- 

 trose into cellulose than into starch ; this may explain the abundant for- 

 mation of starch accompanying tuberization at relatively low temperatures, 

 and the great formation of cellulose associated with elongation at high 

 temperatures. There appears to be a reciprocal relation between leafy 

 shoots and tuberized shoots, any factor tending to suppress the former 

 stimulating the development of the latter ; for example, tuberization is 



