526 SEKSITIVENESS TO GRAVITATION. Chap. XI 



were perfectly straight, and these were placed horizontally on 

 the peat, being covered by a thin layer of it. They were thus 

 left for an average period'of 1 h. 37 m. The tips were then cut 

 off tr^iriKversely for a length of 1-5 mm., and immediately after- 

 wards they were embedded vertically in the peat. In 'his position 

 geotropism would not tend to induce any curvature, but if some 

 influence had already been transmitted from the tip to the part 

 wbich bends most, we might expect that this part would become 

 curved in the direction in which geotropism had previously 

 acted; for it should be noted that these radicles being now 

 destitute of their sensitive tips, would not be prevented by 

 geotropism from curving in any direction. The result was that 

 of the sixteen vertically embedded radicles, four continued for 

 several days to grow straight downwards, whilst twelve became 

 more or less bowed laterally. In two of the twelve, a trace of 

 curvature was perceptible in 3 h. 30 m., counting from the time 

 when they had first been laid horizontally ; and all twelve were 

 plainly bowed in 6 h., and still more plainly in 9 h. In every 

 one of them the curvature was directed towards the side which 

 had been downwards whilst the radicles remained horizontal. 

 The curvature extended for a length of from 5 to, in one in- 

 stance, 8 mm., measured from the cut-oflf end. Of the twelve 

 bowed radicles five became permanently bent into a right angle ; 

 the other seven were at first much less bent, and their curvature 

 generally decreased after 24 h., but did not wholly disappear. 

 This decrease of curvature would naturally follow, if an ex- 

 posure of only 1 h. 37 m. to geotropism, served to modify the 

 turgescence of the cells, but not their subsequent growth to 

 the full extent. The five radicles which were rectangularly 

 bent became fixed in this position, and they continued to grow 

 out horizontally in the peat for a length of about 1 inch dm-ing 

 from 4 to 6 days. By this time new tips had been formed ; and 

 it should be remarked that this regeneration occurred slower in 

 the peat than in water, owing perhaps to the radicles being 

 often looked at and thus disturbed. After the tips had been 

 regenerated, geotropii^m was able to act on them, so that they 

 now became bowed vertically downwards. An accurate draw- 

 ing (Fig. 195) is given on the opposite page of one of these five 

 radicles, reduced to half the natural size. 



We next tried whether a shorter exposure to geotropism 

 would suiBce to produce an after-effect. Seven radicles were 

 extended horizontally for an hour, instead of 1 h. 37 m as in the 



