TUBER. 73 
178. The suis partakes of the nature of the bud. It consists 
of an oval mass of short, thickened scales, closely compacted in 
concentric circles and layers, emitting a stem from their midst, 
and roots from the base or collum (141). 
a. Bulbs are said to be twnicated when they consist of concentric layers, each 
entire, and enclosing all within it, as in the Onion. But the more common 
variety is the scaly bulb, consisting of thickened concave scales, connected to- 
gether at the base, as the lily, tulip. 
b. The bulb is renewed annually, at the approach of winter, by the develop- 
ment of new bulbs in the axils of the scales, which increase at the expense of the 
old. 
c. Bulblets are small, aerial bulbs, formed in the axils of the leaves upon the 
stem, which, when matured, fall to the ground, take root, and produce a perfect 
plant. The tiger-lily (Lilium bulbiferam) is an example, also several species of 
the onion. Such plants are termed bulbiferous. 
FIG. 23.— Forms of the stem;—1, Allium; 2, Arum; 3, Solanum tuberosum; 4, San- 
guinaria ; 5, a spinous branch. 
179. The corm is the dilated, subterranean base of a stem, 
resembling the bulb in form and position, but differing in struc- 
ture, being composed of a uniform and solid mass, without dis- 
tinction of layers or scales. It has been improperly called a 
solid bulb. Ex. Arum, or Indian turnip. 
180. The ruser is an annual, thickened portion of a subter- 
ranean stem, provided with latent buds (called eyes), from 
which new plants arise the succeeding year. It is the develop- 
ment of buds, and the fact of its origin with the ascending axis, 
that places the tuber among stems instead of roots. The pota- 
toe is an example. 
