REPRODUCTION 109 



Another form of underground stem which may be 

 included here is the rhizome of such plants as the iris. 

 It grows partly under the surface of the ground, but its 

 upper side often protrudes and becomes green. It 

 never grows vertically into the air, but its terminal bud 

 sends a shoot upwards which bears the foliage leaves 

 and flowers. It is swollen and filled with food like the 

 potato, but it does not become detached as the tuber 

 does. It is in fact the main stem of the plant and pro- 

 duces only a single bud, instead of a number. 



Two other forms of propagating organ are the bulb 

 and the corm. The bulbs are very large buds, a rela- 

 tively small conical stem being covered with a large 

 number of scaly leaves, the inner ones becoming very 

 succulent and containing food, chiefly sugar, for the 

 young plant, into which the growing apex will develop. 

 The outer leaves remain dry scales and are only pro- 

 tective to the succulent interior. The onion is a good 

 example of the bulb. 



The corm consists of a few internodes of an under- 

 ground stem. It is solid but resembles the bulb in 

 being clothed with dry scaly leaves. There is a bud at 

 its apex which is like the most internal part of the bulb 

 of the onion. An example is afforded by the crocus. 



Roots which are modified for reproductive purposes 

 are shown by the dahlia. They swell and store food 

 after the manner of the tuber, but they do not develop 

 buds. They give up their contents to a shoot put out 

 from a portion of the stem of the original plant. 



Vegetative propagation is very commonly employed 

 by gardeners, by means of cuttings. A piece of a young 

 stem, with a few leaves and buds, cut from the parent 

 will, when planted, often develop roots from the cut 

 surface and so establish a new plant. Some plants pro- 

 pagate themselves naturally in this way ; any detached, 

 injured portion, when left upon damp soil, will put out 



