168 



CLASSIFICATION OF ROOTS AND STEMS 



Storage Stems. — An important office of the stem is the storage of 

 nutriment. All stems perform this office to a greater or less extent, 

 but some are especially modified in form for the purpose. Of these, we 

 have already specially referred to rhizomes and tubers. 



The Bulb. — It remains, then, only to consider the various forms of 

 the bud, including in this term all forms of the bulb. A bulb which, 

 like the onion (Fig. 462), has its fleshy-thickened leaves in the form 

 of broad sheathing organs, seen upon transverse section in the form 

 of concentric rings, is called Tunicated or Coated. Those like the lily 

 (Fig. 461), in which these leaves appear in the form of narrower pro- 

 jecting scales, are called Scaly. When in the axils of the scales we find 



Fig. 461. Scaly bulb of Lilium. 462. Tunicated bulb of onion. 463. Corm of Gladiolus. 464. 

 Axillary bud-bulb of tiger lily. 465. Terminal head of bulbs of onion. 



smaller or secondary bulbs or buds, as in the garlic, it is a Compound 

 bulb. When the texture of a bulb is so dense that its leaf-elements are 

 not conspicuous, it is designated as a Solid bulb. When it is still more 

 dense, as in the case of the Gladiolus (Fig. 463), so that the leaves are 

 not to be distinguished by ordinary methods, it is a Corm. In some 

 plants, the axillary bulbs, instead of occurring in the axils of the bulb- 

 leaves, occur higher up in the axils of the ordinary foliage-leaves, as in 

 the case of the tiger lily (Fig. 464). Their true nature as buds is in 

 this case conspicuously shown, and they are sometimes spoken of as 

 Bud-bulbs. In other related plants, similar bulbs are densely aggre- 

 gated in a terminal umbel looking like an infiorescence, as in some 

 species of onion (Fig. 465). 



