44 THE FLOWERING PLANT. 



tive reproduction is often effected by means of buds, i.e., unex 

 tended shoots. Corms and bulbs are modified underground buds 

 developed by annuals for this purpose. At least one of these 

 structures is produced every year, and very frequently the main 

 corm or bulb develops several smaller ones in its leaf-axils. In 

 the tiger-lily small rounded black bodies may be seen during 

 summer in the axils of the leaves. These are small aerial bulbs, 

 which, sooner or later, fall to the ground and grow into fresh 

 individuals. They are known as bulbils or bulblets. The buds or 

 " eyes" occurring upon tubers, as the potato, serve the same end, 

 and this fact is largely taken advantage of in cultivation. The 

 operation of " budding" may be mentioned here. It bears a 

 close relation to grafting, and consists in the removal of a bud 

 with a small piece of the tissues external to the cambium from 

 one tree, and insertion of the same under flaps cut in the back of 

 another tree. Close contact with the cambium of this tree is 

 thus brought about. 



Motility is exhibited very conspicuously by the stem under 

 various forms. Protoplasmic streamings (cf. p. n) may some- 

 times be seen under the microscope in the cells of some of the 

 hairs which clothe it. This point will be spoken of more fully in 

 a later chapter (p. 72). Larger movements are very frequently 

 met with, especially in growing parts, as is strikingly seen in the 

 case of many twining and other climbing stems. The young 

 shoots of a hop plant, for example, exhibit what is termed circum- 

 nutation, that is to say, they sweep round and round in search of 

 a support, and if they find one, twine round it. Darwin's experi- 

 ments show that in the hop a complete revolution is effected in 

 from two hours to two hours twenty minutes. In a special case 

 noted by him the moving part was about fifteen inches long, and 

 curved in such a way as to describe a circle of nineteen inches in 

 diameter. These data give a maximum rate for the end of the 

 shoot of nearly half an inch per minute. The tip of the minute 

 hand in an ordinary watch, the movement of which can easily be 

 seen, only travels about a quarter as fast as this. 



Irritability and Spontaneity are also possessed by the stem, 

 often in a high degree. As in the case of roots (cf p. 19), 

 gravity exerts an important influence in determining the direc- 

 tion of growth, but the geotropism here is not positive, but 

 negative. The main stem, when strong enough, grows vertically 

 upwards, and its branches have mostly an upward tendency. 

 The experiments with seedlings, described on p. 20, are as 

 instructive here as in the case of the root. Negative geotropism 

 is as necessary to stems for the suitable display of their leaves as 

 positive geotropism is to roots for bringing them into relation 



