THE -GROWTH OF ROOTS AND STEMS. 



185 



223. The bulb only differs from the corm in the relatively 

 smaller size of the stem, and in its investment by thick fleshy 

 leaves, which contain large stores of reserve food. As in the 

 corm, adventitious roots arise from the base of the stem, 

 which usually assumes a discoid shape. In scaly bulbs 

 (Lilies) the fleshy scales, of which the bulk of the bulb is 

 composed, simply overlap at their margins, whereas in tuni- 

 cated bulbs (Onion, Hyacinth) the outer leaves are large and 

 completely ensheathe the inner portions of the bulb. The 

 coloured membranous covering on the outside of such bulbs 

 is formed by the remains of the leaves of a previous season. 



Bulbs, tubers, and corms afford a means of perennation 

 and also of vegetative propagation. They occur most com- 

 monly in the group of Monocotyledons. 



224. Spines. In many cases stems become metamor- 

 phosed into spines which terminate in a hard, sharp point 

 instead of in a, bud or soft growing-point. Many of the 

 branches of Sloe, Hawthorn, etc., be- 

 come modified into special protective 



organs of this nature. That they are 

 really branches which have ceased to 

 grow and whose tips have become 

 hard and sharp-pointed is indicated 

 by the fact that they arise in the 

 axils of leaves, and that they may bear 

 lateral buds or even small foliage- 

 leaves. Spin.es possess a central 

 cylinder of vascular tissue, continuous 

 with that of the stem, as can be seen 

 when they are torn off. 



Spines may also be developed either 

 from entire leaves or from the edge 

 of the blade. The branched spines 



of the Barberry are modified leaves, Fig . 6 i. Branch of Barberry, 

 for in their axils buds are present 

 which may develop into leafy branches 

 or flowering on;s. Moreover, the same 

 Barberry plant. (Fig. 61) often shows intermediate forms 

 between leaves and spines. 



The foliage-leaves of the False Acacia (Eobinia) have a pair 



showing transition of Foliage- 

 leaves into Spines 



