558 SUMMARY AND Chap. Xn. 



gravitation ; for they are thus enabled to move more 

 rapidly and to a greater extent than can most mature 

 plants. Seedlings are subjected to a severe struggle 

 for life, and it appears to be highly important to them 

 that they should adapt themselves as quickly and as 

 perfectly as possible to their conditions. Hence also 

 it is that they are so extremely sensitive to light and 

 gravitation. The cotyledons of some few species are 

 sensitive to a touch ; but it is probable that this is 

 only an indirect result of the foregoing kinds of sen- 

 sitiveness, for there is no reason to believe ^that they 

 proiit by moving when touched. 



Our seedling now throws up a stem bearing leaves, 

 and often branches, all of which whilst young are con- 

 tinually circumnutating. If we look, for instance, at a 

 great acacia tree, we may feel assured that every one of 

 the innumerable growing shoots is constantly describ- 

 ing small ellipses ; as is each petiole, sub-petiole, and 

 leaflet. The latter, as well as ordinary leaves, gene- 

 rally move up and down in nearly the same vertical 

 plane, so that they describe very narrow ellipses. 

 The flower-peduncles are likewise continually circum- 

 nutating. If we could look beneath the ground, and 

 our eyes had the power of a microscope, we should see 

 the tip of each rootlet endeavouring to sweep small 

 ellipses or circles, as far as the pressure of the sur- 

 rounding 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. 



Stems are sometimes developed into long runners or 

 stolons. These circu mnutate in a conspicuous manner, and 

 are thus aided in passing between and over surrounding 

 obstacles. But whether the circumnutating movement 

 has been increased for this special purpose is doubtful. 



