674 



ECOLOGY 



rhizomes, possibly because 

 resistance ; such elongation 



FIG. 989. A tuber of the 

 potato (Solarium tuberosum); note 

 the remains of the rhizome (r) at 

 the end of which the tuber grew 

 and of which it is merely an en- 

 largement; the " eyes " (e) are 

 buds, some or all of which sprout 

 and develop into stems, when the 

 tuber germinates. 



the medium in which they grow offers less 

 favors more rapid migration and the develop- 

 ment of new individuals at a greater distance 

 from the old. Creeping stems appear to be 

 almost as advantageous as rhizomes, since 

 they grow close to the ground in a position 

 favorable for the development of the adven- 

 titious roots that are necessary for propa- 

 gation. In Decodon and Rubus, and even 

 in the strawberry, it is much more difficult 

 for the stems to come into contact with the 

 ground. While most creeping stems re- 

 main alive for some years, as do rhizomes, 

 the internodes of strawberry runners soon 

 die, so that the new potential plants soon 

 become actual individuals. 



Tubers, bulbs, and conns. Tubers. A tuber is essentially a rhizome in which 

 elongation is replaced by stem enlargement, as in the potato, in which the under- 

 ground stem begins as a rhizome, but later develops at the growing 

 tip into a tuber (fig. 989). In Sculellaria parvula and in Juncus 

 Torreyi the rhizome alternately elongates and enlarges, thus de- 

 veloping a beadlike (moniliform) chain of tubers (figs. 990, 983, 

 1069). In nature each tuber usually gives rise to an erect shoot, 

 forming one potential plant ; or an actual plant may result, if the 

 rhizome portions die, as in the potato. As a matter of fact, tubers 

 represent at least as many potential plants as there are buds or 

 "eyes," potato tubers 



commonly being cut " 



into several pieces 

 for planting. Even 

 in tuber fragments 



FIG. 990. The rhizome (r) of a rush (Juncus Torreyi} 

 with tuberized portions (/), from which the erect shoots arise; 

 e, the erect stem of the current season; e', e", the remnants 

 of similar shoots of former seasons. 



without " eyes " a bud may organize and grow into a plant. In one of the 

 yams (Dioscorea saliva), there are aerial tubers, which are of importance in 

 reproduction. 



Bulbs. Bulbs are underground stems differing from tubers and rhizomes in 

 their almost total lack of stem elongation, and in the fact that the main axis is ver- 

 tical and is enclosed by a number of relatively large overlapping scale leaves in 



