Radicles of Sinapis alba.—­The radicles of some plants are indifferent, as far as curvature is concerned, to the action of light; whilst others bend towards and others from it.* Whether these movements are of any service to the plant is very doubtful, at least in the case of subterranean roots; they probably result from the radicles being sensitive to contact, moisture, and gravitation, and as a consequence to other irritants which are never naturally encountered.  The radicles of Sinapis alba, when immersed in water and exposed to a lateral light, bend from it, or are apheliotropic.  They become bent for a length of about 4 mm. from their tips.  To ascertain whether this movement

* Sachs, ‘Physiologie Végétale,’ 1868, p. 44. [page 483]

generally occurred, 41 radicles, which had germinated in damp sawdust, were immersed in water and exposed to a lateral light; and they all, with two doubtful exceptions, became curved from the light.  At the same time the tips of 54 other radicles, similarly exposed, were just touched with nitrate of silver.  They were blackened for a length of from .05 to .07 mm., and probably killed; but it should be observed that this did not check materially, if at all, the growth of the upper part; for several, which were measured, increased in the course of only 8 -9 h. by 5 to 7 mm. in length.  Of the 54 cauterised radicles one case was doubtful, 25 curved themselves from the light in the normal manner, and 28, or more than half, were not in the least apheliotropic.  There was a considerable difference, which we cannot account for, in the results of the experiments tried towards the end of April and in the middle of September.  Fifteen radicles (part of the above 54) were cauterised at the former period and were exposed to sunshine, of which 12 failed to be apheliotropic, 2 were still apheliotropic, and 1 was doubtful.  In September, 39 cauterised radicles were exposed to a northern light, being kept at a proper temperature; and now 23 continued to be apheliotropic in the normal manner, and only 16 failed to bend from the light.  Looking at the aggregate results at both periods, there can be no doubt that the destruction of the tip for less than a millimeter in length destroyed in more than half the cases their power of moving from the light.  It is probable that if the tips had been cauterised for the length of a whole millimeter, all signs of apheliotropism would have disappeared.  It may be suggested that although the application of caustic does not stop growth, yet enough may be absorbed to destroy the power of movement in the upper part; but this suggestion must be rejected, for we have seen and shall again see, that cauterising one side of the tip of various kinds of radicles actually excites movement.  The conclusion seems inevitable that sensitiveness to light resides in the tip of the radicle of Sinapis alba; and that the tip when thus stimulated transmits some influence to the upper part, causing it to bend.  The case in this respect is parallel with that of the radicles of several plants, the tips of which are sensitive to contact and to other irritants, and, as will be shown in the eleventh chapter, to gravitation. [page 484]