The Ash Seed Screw

Abstract

MR. STEPHEN WILSON remarks, in his note on this subject (NATURE, vol. xiii., p. 428), “Why the seed generally becomes twisted as it dries is a very interesting question. But what seems to me the most remarkable fact about this phenomon is, that in every case, and in all trees alike, the thread of the screw is in one direction.” He also alludes to the uniformity in the direction of torsion in the awns of two species of oat. Torsion of all kinds occurring in plants is usually assumed to be due to unequal longitudinal tension (see Sach's “Handbook,” p. 770). In a paper read before the Linnean Society, March 16, I pointed out that the uniformity in the direction of the torsion cannot be thus accounted for; and a totally different explanation was given of the twisting and untwisting of the awns of certain fruits (Avena elatior among the number) when they are dried and moistened. It was shown that the power of torsion resides in the individual cells of which the awn is constructed, and tha it is by their combined action that the awn, as a whole, becomes twisted in drying. It appeared to me extremely probable that the same explanation would hold good for the twisted wing of the ash fruit. I therefore boiled one in nitric acid and chlorate of potassium, by which means the woody tissue is separated into its constituent cells. These were then teazed out on a slip of glass and thoroughly dried over a lamp, and it was found that many of them had become twisted on their axes; and, which is important, that they were all twisted in the same direction as the fruit itself. This artificial drying represents the natural drying process which occurs during the ripening of the fruit. In both cases contraction and consequent torsion result from the loss of water, but in the natural process the cells not being free to twist independently, are compelled to combine in producing that torsion of the whole fruit which we are considering. It is interesting to find the same principle holding good in the case of the ash screw as in that of the awns of various Gramineae and Geraniaceae, and the twisted tails of the achenes in Anemone montana. Moreover, I strongly suspect that the principle of the torsion of an organ being dependent on the twisting of its constituent cells is capable of wider extension, so as to embrace the torsion of the stems of twining plants, &c. This subject I hope immediately to investigate.