THE UPWARD TRANSLOCATION OF FOODS IN WOODY PLANTS. I. TISSUES CONCERNED IN TRANSLOCATION

Abstract

It seems to be very generally believed that in shrubs and trees there is storage of organic matter in the lower part of the trunk and in the roots, and that, as growth starts in the spring, this food becomes soluble and passes upward through the xylem to the developing shoots and leaves. The fact that has been most commonly offered as proof of upward translocation through the xylem is that the xylem tissues of woody plants commonly contain quantities of organic storage products, particularly sugars and starches. It has been shown by Atkins (I9I6), Fischer (I89I), and others that not only the living parenchyma cells but also the water-conducting cells proper, the tracheae and tracheids, contain appreciable quantities of soluble carbohydrates, and Atkins has found these to be present at all seasons of the year. The latter considers that sugars are commonly carried through the xylem. He even goes so far as to say that . . the transference of carbohydrates can no longer be regarded as an occasional and accessory function of the vessels, but is certainly a continual and principal function . . . .. It may well be, however, that there is no flow of solution through the xylem for any great distance, for a possible frequent interposition of living cells across the water-conducting tissues may prevent such a flow. Atkins himself clearly recognizes that water may flow across tissues containing a high solute content without carrying the solutes with it. Experiments performed by Hartig (I858) are also commonly cited in texts as offering proof of upward translocation through the xylem. Hartig ringed a number of trees early in the summer and found early in the following spring that the starch stored below the ring had disappeared. He concluded that the storage products must have been translocated upward through the xylem. It might be, however, that the food stored below the ring was used locally for growth in diameter, or that it was translocated downward through the phloem and was used in root growth. In order to determine more definitely whether the upward translocation of food takes place primarily through the phloem or through the xylem, the writer has conducted a series of experiments part of which are reported in this paper. Some of these experiments also throw light on the matter of food movement from roots or trunks to growing shoots. The discussion therefore naturally falls into two main parts: (i) When upward translocation of organic matter takes place, as it certainly must for at least short distances, does it take place through the water-conducting tissues, the xylem, or