. A textbook of botany for colleges and universities ... Botany. 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
. A textbook of botany for colleges and universities ... Botany. 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 /loryw/a, 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 cell
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