TEGETA'nVE OBGANS OP PLANTS. 389 



extremities of subterranean stems. Tlieir eyes are the 

 points where the buds exist, usually three together, 

 and where minute scales — rudimentary leares — may be 

 observed. The common potato and artichoke {Helian- 

 fhus tuberosus) are instances of this kind of tubers. 

 Tubers serve excellently for propagation. Each eye, or 

 bud, may become a fiew plant. From the quantity of 

 starch, etc., accumulated in them, they are of great 

 importance as food. The number of tubers produced by 

 a potato-plant appears to be increased by planting orig- 

 inally at a considerable depth, or by "hilling up" earth 

 around the base of the aerial stems during the early 

 stages of its growth. 



Bulbs are greatly thickened stems, whose leaves — 

 usually having the form of fleshy scales or concentric coats 

 — are in close contact with each other, and arise from 

 nearly a common base, the internodes being undeveloped. 

 The bulb is, in fact, a permanent bud, usually in part 

 or entirely subterranean. From its apex, the proper 

 stem, the foliage, etc., proceed; while from its base 

 roots are sent out. The structural identity of the bulb 

 with a bud is shown by the fact that the onion, which 

 furnishes the commonest example of the bulb, often 

 bears bulblets at the top of its stem, in place of flowers. 

 In like manner, the axillary buds of the tiger-lily are 

 thickened . and fleshy, and fall off as bulblets to the 

 ground, where they produce new plants. 



Stbuctuee of the Stem. — The stem is so compli- 

 cated that to discuss it fully would occupy a volume. 

 For our immediate purposes it is, however, only neces- 

 sary to notice ite structural composition very concisely. 



The rudimentary stem, as found in the seed, or. the 

 neWrformed part of the maturer stem at the growing 

 points just below the terminal buds, consists of cellular 

 tissue, ov is an aggregate of rounded and cohering cells, 

 which rapidly multiply during the Tigorous growth of 

 the plant. 19 



