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Show 184 SENSITIVENESS OF TIIB APEX 0IIAP.'!II. them not quite perpendicularly downwards, were coated with the black grease for a length of from 1 to U mm. The sieves were inclined at 30° with the horizon. The greater numhor of these radicles were examined after 22 h., and a few after 25 h., and within these intervals they had grown so quickly as to have nearly doubled their lengths. With the ungreased radicles the chief scat of curvature is at a distance of not less than between 3·5 and 5·5 nun., and not more than between 7 and 10 mm. from the apex. Out of the 13 radicles with greased tips, 4 had not moved at all towards the sieve; G were deflected towards it and from the perpendicular by angles varying between 10° and 35°; and 3 had come into close contact with it. It appears, therefore, at first sight that greasing the tips of these raclicles had checked but little their bending to the adjoining damp surface. But the inspection of the sieves on two occasions produced a widely different impression on the mind; for it was impossible to behold the radicles with the black greased tips projecting from the bottom, and all those with ungreased tips, at least 40 to 50 in number, clinging closely to it, and feel any doubt that tho greasing had produced a great effect. On close examination only a single ungreased radicle could he found which had not become curved towards tho sieve. It is probable that if the tips had been protected by grease for a lo11gth of 2 mm. instead of from 1 to U mm., they would not have been a:lfectcu by the moist air and none would have become curved. 'l'riticttm vulga1·e.-Analogous trials were ma<le on 8 raclicles of the common wheat; an<l greasing their tips produced much less effect than in the case of the oats. After 22 h., 5 of them had come into contact with the bottom of the sieve; 2 had moved towards it 10° and 15° and one alone remained perpendicular. Not one of tho ve:.y numerous ungrcased radicles failed to come into close contact with the sieve. These trials were made on Nov. 2~th, when the temperature was only 4o·s C. at 10 A.M. We should hardly have thought this case worth notice, had it not been for the following circumstance . . In the beginning of October, when the temperatme was cemndembly higher, viz., 12° to 13° C., we found that only a few of tl:e ungreased radicles beca.me bent towards the sieve; and this jnclicates that sensitiveness to moisture in the air is increased d. 1 of by a low temperature, as we have seen with the ra ICes . Vicia faba relatively to objects attached to their tips. But Ill tho present instance it is possible that a difference in the dryness CHAP. III. OF THE RADICLE TO MOIST AIR. 185 of the air may have caused the difference in the results at the two periods. Finally, the facts just given with respect to Phaseolus multijlorus, Vicia faba, and Avena sativa show, as it seems to us, that a layer of grease spread for a length of 1! to 2 mm. over the tip of the radicle, or the destruction of the tip by caustic, greatly lessens or quite annuls in the upper and exposed part the power of bending towards a neighbouring source of moisture. We should bear in mind that the part which bends most, lies at some little distance above the greased or cauterised tip; and that the rapid growth of this part, proves that it has not been injured by the tips having been thus treated. In those cases in which the radicles with greased tips became curved, it is possible that the layer of grease was not sufficiently thick wholly to exclude moisture, or that a sufficient length was not thus protected, or, in the case of the caustic, not destroyed. When radicles with greased tips are left to grow for several days in clamp air, the grease is drawn out into the finest reticulated threads and dots with narrow portions of the surface left clean. Such portions would, it is probable, be able to absorb moisture, and thus we can account for several of the radicles with greased tips having become curved towards the sieve after an .interval of one or two days. On the whole, we may mfer that sensitiveness to a difference in the amount of moisture in the air on the two sides of a radicle resides in the tip, which transmits some influence to the upper part, causing it to bend towards the source of moisture. Consequently the movement is th. e reverse of that caused by objec' ts attached to one 8b1~e of the tip, or by a thin slice being cut off or by e.m g sl 1' g h tly cauterised. In a future chap' ter it . Will be shown that sensitiveness to the attraction of .: ,:1,:,:;.; , t,::~; ,., •• 1' |
