Net production of diffusible auxin by fronds of O. cinnamomea has been found to extend throughout the period of final expansion, with a maximum which occurs shortly after the rate of elongation has reached its maximum and begun to decline. Auxin is apparently produced only in the pinnae, and from these it enters the rachis where its movement is polar. Diffusible auxin moves through the rachis without significant loss except in the region of extensive elongation where a certain amount disappears. No such disappearance was found in ether extraction studies. Moreover, in certain cases, ether-extractable auxin persists for as much as 48 hours in the rachis after removal of the auxin source, although none can be obtained by diffusion under these conditions. It is concluded that diffusible auxin is more closely related to growth phenomena in the frond than is extractable auxin. In many cases, there is an apparent failure of transport in the leaf base which reduces or eliminates expected auxin yields by diffusion at this level. Diffusible auxin is an essential participant in three morphogenetic phenomena in the developing frond: rachis elongation, crozier uncoiling, and final differentiation of xylem and hypodermal sclerenchyma. In none of these can it be considered as a specific determining factor since the response to auxin depends upon the physiological state of the reacting cells. In the intact frond diffusible auxin is present in excess of the minimum required for both rachis elongation and uncoiling. Several aspects of the relationship between auxin distribution and development in the frond are discussed.
Copyright (C) 1960, Society for Experimental Biology
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