The immediate cause of the mechanically weak unions that occur in some incompatible trees is by now well known. They are due to the abnormal arrangement of the xylem tissue, particularly the fibres, which, instead of interlocking across the union, either curve in a horizontal direction or are separated by a layer of parenchymatous tissue. This has been well described by Proebsting (1928), Bradford and Sitton (1929), Eames and Cox (1945), Herrero (1951) and others. All are agreed that the discontinuity of the vascular tissues is due to some interruption, temporary or permanent, of normal cambial activity at the point of union, but there is as yet little information regarding the basic causes of this interruption. Most anatomical investigations of the union have been concerned primarily with the structure of the woody cylinder, although, as long ago as 1929, Bradford and Sitton wrote, “ the break-down in the bark seems more consistent, more complete and perhaps more important than that in the wood ”. The first attempt to explain the break in cambial continuity was made by Herrero (1951) who concluded from his anatomical investigations that there existed in incompatible combinations a latent antagonism between adjoining cells of stock and scion, and that spontaneous breaks occurred in the cambial layer at the point of union, possibly in connection with changes undergone at the union by substances elaborated in the leaves. Thiel (1954) agrees that the underlying causes of pear/quince incompatibility are probably biochemical in nature. It is believed that further evidence for this view is provided by observations which have been made at East Mailing on the histology of the bark tissue in some pear/quince unions and which will be described in this paper.
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Mosse et al. (1956) studied this question.