44 THE FLOWERING PLANT. 
tive reproduction is often effected by means of buds, 7.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 aérial 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. 11) 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 1s 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 czrcwm- 
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 
