72 Chapters in Modern Botany chap. 



houses of food, will be raised above the ground and ex- 

 panded in the air. Sooner or later the arch straightens 

 into an upright shoot. Both in the young shoot and in the 

 expanded cotyledons there is power of movement ; they are 

 exquisitely sensitive to light and to gravitation. 



The power of movement does not cease when the shoot 

 becomes a stem with leaves and branches. " If we look, 

 for instance, at a great Acacia tree, we may feel assured 

 that every one of the innumerable growing shoots is con- 

 stantly describing small ellipses ; as is each petiole, sub- 

 petiole, and leaflet. The latter, as well as ordinary^ leaves, 

 generally move up and down in nearly the same vertical 

 plane, so that they describe very narrow ellipses. The 

 flower-peduncles are likewise continually circumnutating. 

 If we could look beneath the ground, and our eyes had the 

 power of a microscope, we should see the tip of each root- 

 let endeavouring to sweep small ellipses or circles, as far as 

 the pressure of the surrounding earth permitted. All this 

 astonishing amount of movement has been going on year 

 after year since the time when, as a seedling, the tree first 

 emerged from the ground." 



Methods of Observation. — But how can these move- 

 ments be seen and measured ? In some of the more marked 

 cases, as of twiners and climbers, there is no difficulty what- 

 ever. We need only observe the position of the plant at 

 intervals of hours and days. Or if we note the position of 

 a growing shoot in the garden as it bends over to one side, 

 and fix a piece of string, by means of a couple of stakes, 

 so that it lies in a line with the shoot when looked at 

 from above, we can, if we return in half an hour or 

 so, plainly see that the shoot has moved through a large 

 angle. 



But there are of course more delicate modes of observa- 

 tion. Darwin gives many figures representing, e.g. the 



