STEMS 



675 



which food accumulates (as in the lilies, fig. 991) Sometimes, as in the tiger lily 

 (fig. 992) and in Lycopodium, bulbs develop on aerial stems; in some species of 

 Sedum, ordinary axillary buds drop off and develop into independent plants. Bulbs 

 usually give rise to a single potential plant. 



Corms. Corms usually agree with bulbs in having vertical orientation, slight 

 stem elongation, and prominent apical buds, while they agree with tubers in their 

 inconspicuous scale leaves and in the accumulation of food in the stem (fig. 993). 

 Transitions exist between the various kinds of under- 

 ground stems, for example, in Trillium. 



The advantages of tubers, corms, and bulbs. Tubers, 

 corms, and bulbs are much inferior to rhizomes and run- 

 ners as propagative organs, particularly because their slight 

 elongation limits the number of potential plants developed 

 and permits but slight migration from the original center. 

 The advantages of these organs are found rather in food 

 accumulation (p. 719) and in protection (p. 716). When 

 a tuber (as in the potato) or a bulb (as in Erythronium) 

 is borne at the end of a rhizome, the possibilities of 

 effective migration and reproduction are unsurpassed, but 

 this is due chiefly to the rhizome element in the under- 

 ground stem, the tuberous or bulbous portion being of 

 significance mainly in furnishing an abundant supply of 



FIG. 99 1. A median 

 longitudinal section 

 through the bulb of a 

 hyacinth (Hyacinthus 

 onentalis), showing the 

 overlapping scale leaves 

 (s) which compose the 

 bulb; the innermost 

 leaf primordia develop 

 into foliage leaves (/); 

 /, flower stalk ; (r), ad- 

 ventitious roots. 



FIG. 992. A portion of a shoot of the tiger lily (Lilium 

 tigrinum), showing the bulbils (b) which develop in the axils 

 of sessile foliage leaves; these bulbils readily become detached 

 and fall to the ground. 



food to the new plant. Tubers, corms, and bulbs, like rhizomes, have a definite 

 soil level varying with the species, the largest forms commonly having the greatest 

 depth. Because of their slight migratory powers it takes these organs much longer 

 than rhizomes to reach the proper level when displaced ; however, in such forms as 

 Erythronium the rhizomes (sometimes called droppers') which bear the new bulbs 

 grow up or down as the case may be, placing the new plant at the proper level (figs. 

 718, 719). In Erythronium albidum there has been recorded a descent of ten 

 centimeters in one season. The soil position of tubers, corms, and bulbs is be- 

 lieved to depend chiefly upon their distance from the synthetic organs, and to a 

 smaller degree upon variations in soil moisture. 



