Chap. III. SUMJIAEY OF CIIAPTEK. 197 



apex (protected by the root-cap) being pressed down 

 by the longitudinal expansion or growth of the ter- 

 minal rigid portion, aided by its transverse expan- 

 sIdd, both of which forces act powerfully. It is, 

 however, indispensable that the seeds should be at 

 first held down in some manner. When they lie 

 on the bare surface they are held down by the attach- 

 ment of the root-hairs to any adjoining objects ; and 

 this apparently is effected by the conversion of 

 their outer surfaces into a cement. But many seeds 

 get covered up by various accidents, or they fall into 

 crevices or holes. With some seeds their own weiglit 

 suffices. 



The circumnutating movement of the terminal grow- 

 ing part both of the primary and secondary radicles 

 is so feeble that it can aid them very little in pene- 

 trating the ground, excepting when the superficial 

 layer is very soft and damp. But it must aid them 

 materially when they happen to break obliquely into 

 cracks, or into burrows made by earth-worms or larvae. 

 This movement, moreover, combined with the sen- 

 sitiveness of the tip to contact, can hardly fail to be 

 of the highest importance; for as the tip is always 

 endeavouring to bend to all sides it will press on all 

 sides, and will thus be able to discriminate between 

 the harder and softer adjoining surfaces, in the same 

 manner as it discriminated between the attached 

 squares of card-like and thin paper. Consequently it 

 will tend to bend from the harder soil, and will thus 

 follow the lines of least resistance. So it will be if it 

 meets with a stone or the root of another plant in the 

 soil, as must incessantly occur. If the tip were not 

 sensitive, and if it did not excite the upper part of the 

 root to bend away, whenever it encountered at right 

 angles some obstacle in the ground, it would be liable 



