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Show 196 SUMMARY OF CHAPTER. CHA1'. III. when their tips were slightly cauterised on the lower side. Finally, the several co-ordinated move~ents by which radicles are enabled to perform then proper functions are admirably perfect. In whatever direction the primary radicle first protrudes from the seed, geotropism guides it perpendicularly downward~; and the capacity to be acted on by the attraction of gravity resides in the tip. But Sachs ~as proved • that the secondary radicles, or those emitted by the primary one, are acted on by geotropis~ in such a manner that they tend tq bend only obhquely downwards. If they had been acted on like the primary radicle, all the radicles would have penetrated th.e ground in a close bundle. ~Ve ~ave seen that. 1f the end of the primary ra(hcle IS cut off or mjured the adjoininO' secondary radiclos become geo· tropi~ and grow v~rtically downwards. This power must often be of great service to the plant, when the primary radicle has been destroyed by t~e larvm of insects, burrowing animals, or any other accident. The tertiary radicles, or those emitted by the secondary ones are not influenced, at least in the case of the bea~, by geotropism; so they grow out freely in. all directions. From this manner of growth of the vanous kinds of radicles, they are distributed, together wi~h their absorbent hairs, throughout the surrounding soil, as Sachs has remarked, in the most advantageous manner; for the whole soil is thus closely searched. Geotropism, as was shown in the last chapt.er, excites the primary radicle to bend downwards with very little force, quite insufficient to penetrat~ the ground. Such penetration is effected by the pomted * 'Arbeiten Bot. Institut., Wiirzburg,' II~:ft iv. 1874, PP· 60j-GS1. CHAP. III. SUMMARY OF . CHAPTER 197 apex (protected by the root-cap) Laing pressed down by the longitudinal expansion or growth of tho terminal rigid portion, aided by its transverse expansion, 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 arc held down by the attachment 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. \Vith some seeds their own weight suffices. The circumnutating movement of tho terminal growing part both of the primary and secondary raclicles is so feeble that it can aiel them very little in penetrating the ground, excepting when the superficial layer is very soft and damp. But it must aid them materially when they happen to br ak obliquely into cracks, or into burrows made by earth-worms or larvre. This movement, moreover, combined with the sensitiveness of the tip to contact, can harclly fail to be of the highest importance; for as the tip is always e?deavouring to bond to all sides it will press on all Bides, and will thus be able to discriminate between the harder and softer adjoining surfaces, in the same manner as it discriminated between the attached s~uares 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 m~ets with a stone or the root of another plant in the Boll,. ~s must incessantly occur. If the tip were not sensttive, 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 |