WERY little is certain as to the movements of carbohydrates in the plant. It is generally agreed that the green plant can build them up for its own needs in leaves exposed to the light, and that these supplies are then utilised in growth throughout the plant, so that considerable move ments of sugars must take place from the leaves to the roots and fruits and various storage organs. There is no agreement, however, as to the tissue through which this movement takes place. Only two tissues, regularly present in this plant axis, are so extended in the longitudinal direction as to be very likely to convey such substances for long distances through the axis; these are the wood or xylem, and the phloem or bast. Usually, the sieve-tubes of the phloem have been regarded as the channels of sugar transport, as micro-chemical observations, such as those of Prof. Mangham, seemed to show considerable quantities of sugar in these tissues. The phloem in many trees is confined to a narrow layer near the periphery, so that it is possible to cut this channel completely by removing a narrow strip of tissue from the outside of the stem, and there is evidence that such ringing experi ments always interfere with carbohydrate trans port. Prof. H. H. Dixon pointed out, however (NATURE, vol. 110, 547-551; 1922>, that the xyle sap usually contains appreciable quantities of sugar, and that in the ringing experiment it is very difficult to remove the phloem without doing some damage to the wood. As a result, the wood may be partially blocked, so that the interruption of the carbohydrate movement, attributed to the ringing of the phloem, may be really due to the partial choking of the xylem channels.
"Memoirs of the Cotton Research Station, Trinidad, Series B, Physiology, No. 1. Studies on the Transport of Carbohydrates in the Cotton Plant". By T. G. Mason and E. J. Maskell . Empire Cotton Growing Corporation, 2 Wood Street, Millbank, London, S.W.1. 1928.