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Show 452 SENSITIVENESS TO LIGII'r. CnAr.IX. taken by E-ach semicircle, within a limit of error of at most 5 minutes. Although the rato of movement in different parts of the same revolution varied greatly, yet 22 semicircles to tho light wore completed, each on an average in 73·95 minutes; and 22 semicircles from the light each in 73·5 minutes. It may, there· fore, be said that thoy travelled to anJ from tho light at exactly the same average rato; though probably the accuracy of tho result was in part accidental. In the evening the stems were not in the least deflected towards the window. N cvnrtheless, there appears to exist a vestige of heliotropism, for with 6 out of the 7 plants, the :first semicircle from tho light, described in the early morning after they had boon subjected to tlarkness during the night and thus probably rendered more sensitive, required rather more time, and the first semicircle to the light consiclorably loss time, than the average. Thus with all 7 plants, taken toge~her, the mean time of the first semicircle in tho mornmg from the light, was 76·8 minutes, insteaJ of 73·5 ~inutes, which is the mean of all the semicircles dunng the day from the light; and the moan o.f the first s~~~: circle to the light was only 63·1, mstead o~ !3 9J minutes, which was the moan of all the semlClrcles during the clay to the light. . . Similar observations were made on 1Yistaria &nensls, and the mean of 9 semicircles from the light was 117 minutes and of 7 semicircles to the light 122 minutes and' this difference does not exceed the pro· bable li:Uit of error. During the three clays of expo· sure the shoot di. d not become a t a1 1 be n t towards fit het w.m d'o. w before whw. h I. t stoo J · I n this case thef rsh semicircle from the Iio·ht in the early morning 0 er day required rather l~ss time for its performa.nce tl~;1 diJ 'the :first semicircle to the light; and this resu ' CIIAP. IX. SENSITIVENESS TO LIGHT. 453 if not accidental, appears to illflicato that the shoots retain a trace of an original apholiotropic tendency. With Lonicera brachypoda the semicircles from and to the light differed considerably in time; for 5 semicircles from the light required on a mean 202·4 minutes, and 4 to the light, 229·5 minutes; but the shoot moved very irregularly, and under these circumstances the observations were mnch too few. It is remarkable that the same part on the same plant may be affected by light in a widely different manner at different ages, and as it appears at different seasons. The h ypocoty leclonous stems of Ipomma cr:erulea and purpurea are extremely heliotropic, whilst the stems of older plants, only about a foot in height, are, as we have just seen, almost wholly insensible to light. Sachs states (and we have observed the same fact) that the hypocotyls of tho Ivy (Hedera helix) are slightly heliotropic; whereas the stems of plants grown to a few inches in height become so strongly apheliot~ opic, that they bend at right angles away from the light. Nevertheless, some young plants which had behaved in this manner early in the summer again became distinctly heliotropic in the beginning of September; and the zigzag courses of their stems, as they slowly curved towards a north-east window, were traced during 10 days. The sterns of very young plants of TroplR.olum majus are highly heliotropic, whilst those of older plants, according· to Sachs are s)io-htly h r ' b ap e IOtropic. In all these cases the heliotropism of the very young stems serves to expose the cotyledons or wh e n t he cotyledons arc hypogean the :first true' leaves, fully to the light; and the loss of this power ~y the older stems, or their becoming apheliotropic, 18 connected with their habit of climbing. Most seedling plants are strongly heliotropic, and |
