STUDIES OF THE CLOVE TREE

Abstract

A detailed study was made of the histology of apparently healthy clove trees and of trees expected to die from sudden death disease, in an attempt to find either a diagnostic character of the disease or some indication of a possible pathogen.The apparently healthy clove tree differs from a typical dicotyledon in a number of ways. This is especially true of the vascular system where, not only are the bundles bicollateral, but, in the stem, secondary thickening occurs in the internal phloem, necrosis of the first formed cells keeping pace with the production of new cells. A definite cambium is found only to the exterior of the xylem and this is exceptionally narrow and the cells have very delicate walls. Phloem necrosis may occur in any part of the tree. The development and lignification of the xylem is very uneven, but this seems to bear no relation to the season. Tyloses are formed in the vessels of the root and, to a less extent, in those of the stem. Starch reserves are usually small, but large quantities of tannins, oils and calcium oxalate are secreted in all tissues. The oil may be contained in special schizogenous glands which are abundant in the aerial parts of the tree. Much mechanical tissue, consisting of fibres and stone cells, is formed in the pith and cortex of the stem and in the phloem in all parts of the tree, but lignification of this tissue is very uneven. Occasional dead cells are found in all tissues, even in the primary meristem. In the leaf, the plastids develop very unevenly; this is followed by retarded development of the pigment in some parts, resulting in a flecked appearance. Stomata are exceptionally numerous; in the leaf, they are confined to the lower surface and always communicate with very small cavities. Air spaces in the mesophyll are very small and are almost non‐existent in other tissues. Bodies, morphologically similar to the amorphous intracellular inclusions due to some viruses but differing chemically from them, are abundant in the leaves.Some of these characters are usually associated with a diseased condition, but they occur in both healthy and diseased cloves in the Zanzibar Protectorate. In the sudden death suspect, some may occur earlier or be more accentuated than in the healthy tree. This evidence alone might suggest that sudden death is merely the culmination of a long period of adverse conditions, but other evidence suggests very strongly that the disease is due to a pathogen. Of the groups of organisms which may cause disease, only fungi were found. These occurred in the absorbing roots but are unlikely to be the pathogens causing sudden death, for, until the tree has actually wilted, very few hyphae are present, and no more than are found in healthy trees. Nothing incompatible with the suggestion that sudden death is caused by a virus was found.