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Show 182 SENSITIVENESS OF THE APEX CHAP. III. considerably curved towaras the sieve ; and the absence of curvature in some of the others might perhaps be accounted for by their not having grown very well. But it should be observed that during tho first 19 h. to 24 h. all grew well ; two of them having increased 2 and 3 mm. in length in 11 h.; five others increased 5 to 8 mm. in 19 h. ; and two, which had been at first 4 and 6 mm. in length, increased in 24 h. to 15 and20 mm. The tips of 10 radicles, which likewise grew well, were coated with tho grease for a length of only 1 mm., and now the result was somewhat different; for of these 4 curved themselves to tho sieve in from 21 h. to 24 h., whilst 6 did not do so. Five of the latter were observed for a.n additional day, and now all excepting one became curved to the sieve. The tips of 5 radicles were cauterised with nitrate of silver, and about 1 mm. in length was thus destroyed. They were observed for periods varying between 11 h. and 24 h., and were found to have grown well. One of them had curved until it came into contact with the sieve ; another was curving towards it; whilst the remaining three were still vertically dependent. Of 7 not cauterised radicles observed at the same time, all had come into contact with the sieve. The tips of 11 radicles were protected by moistened goldbeaters' skin, which adheres closely, for a length varying from H to 2~ mm. After 22 h. to 24 h., 6 of these radicles were clearly Lent towards or had come into contn.ct with the sieve; 2 were slightly curved in this direction, and 3 not at all. All had grown well. Of 14 control specimens observed at the same time, all excepting one bad closely approached the sieve. It appears from these cases that a cap of goldbeaters' skin checks, though only to a slight degree, the bending of the radiclcs to an adjoining damp sUTface. Whether an extremely thin she~t of this substance when moistened allows moisture from the au to pass through it, we do not know. One case indicated that the caps were sometimes more efficient than appears from tbo above results; for a radicle, which after 23 h. had only slightly approached the sieve, had its cap (H mm. in length) removed, and during the next 15~ h. it curved itself abruptly towards the source of moisture, the chief seat of curvature being at a distance of 2 to 3 mm. from the apex. . Vicia faba.-The tips of 13 radicles were coated w1th the grease for a length of 2 mm.; and it should be remembered that with these radicles the seat of chief curvature is about CHAP. III. OF THE RADICLE TO MOIST AIR. 183 4 or 5 rom. from the apex. Four of them were examined after 22 h., three after 26 h., and six after 3G h., n.ncl none had been attracted towards the damp lower surface of the sieve. In another trial 7 radicles were similarly treated, and 5 of them still pointed perpendicularly downwards after 11 h., whilst 2 were a little curved towards the sieve ; by an accident they were not subsequently observed. In both these trials the radicles grew well ; 7 of them, which were at first from 4 to 11 rom. in length, were after 11 h. between 7 and 16 mm.; 3 which were at first from 6 to 8 mm. after 26 h. were 11·5 to 18 mm. in length; and lastly, 4 radicles which were at first 5 to 8 mm. after 46 h. were 18 to 23 mm. in length. The control or ungreased radicles wore not invariably attracted towards the bottom of the sieve. But on one occasion 12 out of 13, which were observed for periods between 22 h. and 36 h., were thus attracted. On two other occasions taken together, 38 ont of 40 were similarly attracted. On another occasion only 7 out of 14 behaved in this manner, but after two more days the proportion of the curved increased to 17 out of 23. On a last occasion only 11 out of 20 were thus attracted. If we add up these numbers, we find that 78 out of 96 of the control specimens curved themselves towards the bottom of the sieve. : Of the specimens with greased tips, 2 alone out of the 20 (but 7 of these were not observed for a sufficiently long time) thus curved themselves. We can, therefore, hardly doubt that the tip for a length of 2 mm. is the part which is sensitive to a moist atmosphere, and causes the upper part to bend towards its soUTce. The tips of 15 radicles were cauterised with nitrate of silver a~d they grew as well as those above described with greased tips. After an interval of 24 h., 9 of them were not at all ~urved towards the bottom of the sieve; 2 were cUTved towards It at angles of 20° and 12° from their fo1·mer vertical position, a.nd 4 had come into close contact with it. Thus the destructiOn of the tip for a length of about 1 mm. prevented the curvature of the greater number of these radicles to the adjoining d.amp surface. Of 24 .control specimens, 23 were bent to the Sieve, and on a second occasion 15 out of 16 were similarly ~urved in a greater or less degree. These control trials are Included in those given in the foregoing paragraph. Avena sativa.-The tips of 13 radicles which projected between 2 and 4 mm. from the bottom of the sieve, many of ,. , .~ , e•" '.: .r'!.,.;".; ~ .; .:)1 11.1 •• t::::l |