Henry H. Dixon

Transport of Organic Substances in Plants

THE older writers and modern text-books affirm that the organic materials (carbohydrates, etc.) manufactured in the leaves of plants are transported downwards by means of the bast through their organs to places of consumption and storage. This belief seems to be based entirely on ringing experiments. A priori the bast appears to be very unsuitable for carrying out this function. Even in the most rapidly assimilating plants its cross-section is small. It is formed of short cells and comparatively short, narrow tubes, so that many cross-partitions must be traversed by the stream carrying these organic substances if they use it as a conduit. Furthermore, its resistance must be greatly increased by the fact that a large proportion of its cross-section is occupied by viscid contents—protoplasm and proteins. Evidently, in such a conduit we could only expect that velocities of transport comparable with diffusion velocities could be attained. Assuming that a 10 per cent, solution of sucrose were supplied by the leaves and that this was completely converted into an insoluble carbohydrate in a storage organ 50 cm. distant, then we might expect, after a steady state had been attained, a rate of transport, from diffusion alone, of about 2 milligrams per sq. cm. per diem. This would be equivalent to a 10 per cent. solution moving at the rate of 0.2 mm. per diem. Although this diffusion rate of transport might be somewhat accelerated by protoplasmic streaming, it is quite evident that diffusion in the bast is inadequate to account for the observed rate of transport of carbohydrates in plants. The insufficiency of diffusion to transport carbohydrates is strikingly borne out by those experiments in which cut floating leaves exposed to conditions suitable for photosynthesis accumulate carbohydrates, while only negligible quantities find their way into the water.

On the Ascent of Sap.

Strasburger’s experiments have eliminated the direct action of living protoplasm from the problem of the ascent of sap, and have left only the tracheal tissue, as an organised structure, and the transpiration-activity of the leaf, wherein to seek an explanation of the phenomenon. The authors investigate the capability of the leaf to transpire against excessive atmospheric pressures.

On the structure of coccospheres and the origin of coccoliths

At the beginning of September last year, I visited Valencia, Co. Kerry. It occurred to me there that coccospheres might possibly be drifted in on the warm current of the Gulf Stream, which impinges on the south-west coast of Ireland, and as they float in would become entangled in the sea-weeds on the coast. With this idea, I gathered some of the finer marine algae, such as species of Cladophora, Polysiphonia, and Plocamium, &c., from the rock pools in Valencia Harbour. Taking care to wash as little of the silt or sediment as possible from them, I fixed the mass in dilute formalin.

Bast-Sap in Plants

IN 1858 Th. Hartig directed attention to the fact that many trees yield drops of sap when their bark is punctured in summer and autumn. This sap issuing from the inner layers of the bast was said to include small quantities of nitrogenous substances with large quantities of various kinds of sugar. Obviously its motion and constitution might yield some information on the transport problem of organic substances. We found the most abundant flow when the puncture reaches the inner layers of the bast; deepening the puncture so as to reach the wood, causes the sap exuded to be instantly drawn in by the tensile water in the vessels. Hence the exuded sap is not driven out by root-pressure through the outer xylem. The exuding sap is transmitted by the sieve-tubes as we demonstrated by forcing a solution of potassium ferrocyanide into the puncture and afterwards tracing its path by means of ferric chloride.

Coccoliths in our Coastal Waters

IN our communication to NATURE, September 16, 1897, we say “the presence of these bodies (coccoliths) in our coastal waters does not appear to have been recorded.” Since this was written we find that Dr. Wallich, in the Ann. and Mag. of Natural Hist., vol. ii. 1868. p. 319, stated “Coccospheres have been met with by me profusely … in material collected at the surface of the open seas of the tropics, and also in dredgings from shoal water off the south coast of England”

A Binocular Illusion

IF an observer views a piece of linoleum, or pavement, marked out as a checkerboard in light and dark squares, it is possible by causing the lines of vision of the two eyes to converge, that is, locating the fixation point, above the linoleum, to produce the impression that the pattern is raised above the level of the floor, and that it is on a smaller scale than that of the actual linoleum. This occurs when the images of two neighbouring dark squares fall on the corresponding points of the two retinas. A still smaller pattern at a still higher level may be produced when the images of two dark squares, separated actually by two light and one dark square, are caused by suitable convergence to fall on corresponding points of the two retinas.

Prof. J. Joly, F.R.S

JOHN JOLY came of a remarkable lineage. His fathers grandfather was a member of a French noble family. His mother, a German countess, whose family had been ennobled by Frederick the Great, was descended from Greek, Italian, and English ancestors. This mixture of blood, perhaps, may explain his ready sympathy with the most diverse personalities, his princely generosity which often gave to others what he denied to himself, and his versatility which enabled him to prosecute research in so many fields of knowledge, and to obtain æsthetic pleasure in the realms of art, literature, iusic and science.

Floating Mercury on Water

IN a letter appearing in NATURE for Mar. 16, Mr. N. K. Adam describes floating mercury droplets. These droplets are minute (0.5 mm. in diameter), and Mr. Adam evidently regards them as fluid throughout supported by the surface tension of the water.

Transmission of Stimuli in Plants: A Correction

IN a communication dated May 22, Prof. Ricca asks me to point out that there is an error in his letter as printed in NATURE of May 8, p. 655. There it is stated that the closing of the pinnules from base to apex of a pinna indicated times of transmission of the stimulating substance from the base to the apex of the pinna of 5, 4, 3½, 2½, 2 min. This of course should read, 5, 4, 3½, 2½, 2 sec.