703 



find a case of healing. The difficulty in closing the wound is explained by 

 the presence of new gum centres in the callus. 



We have the following points to emphasize from the consideration of 

 the cherry branch affected by gummosis here illustrated, i. The pro- 

 duction of parenchymatous tissue groups between the prosenchymatous 

 elements of the wood. 2. The position of the groups between two medul- 

 lary rays which can curve about the parenchyma aggregations, and, more 

 rarely, are able to participate in their formation. 3. The production of 

 these groups independently of wounds. 4. The liquefaction of these tissue 

 aggregations into gum pockets into which the resistant medullary ray cells 

 grow like threads. The last circumstance is explained by the fact that in the 

 same cambial ring zone of the branch, or trunk, the medullary cells develop 

 more rapidly than the tissue lying between them, and, therefore, are elon- 

 gated further radially into the bark body where they function as parenchy- 

 matous tissue. At the time when the process of liquefaction begins, the 

 medullary ray cells, therefore, are tougher and more resistant and the first 

 gummy centres appear as holes between two medullary rays when the 

 gummosis is not caused by wounds. 



The more recent experiments attempting to explain the production of 

 gum exudation^ begin with the phenomena of injury. Beijerinck and Rant- 

 assert in their very thorough work that the gummy exudation depends "on 

 the abnormal development of the embryonic wood tissue caused by the 

 wound stimulus." 



Beijerinck presents the subject thus : the normal plant forms cytolytic 

 substances which take part in the formation' of ducts and tracheids. The 

 physiological gum, thus produced, is in fact usually entirely re-absorbed, yet, 

 under certain circumstances, it remains demonstrable as such even in the 

 cavities of the mature ducts. The "gummy exudation, therefore, depends 

 upon an abnormal increase of the action of those cytolytic substances under 

 the influence of dying cells, perhaps because an especially large number of 

 these are produced in necrobiosis. By necrobiosis is meant the cell activity 

 after the death of the protoplasm, while the enzyme bodies remain active." 



Ruhland^ opposes this theory. He calls attention first of all to the fact 

 that gummosis can take place in seeds, fruits*, leaves and also, in the 

 phellogen, on which last point he lays especial stress. He found considerable 

 masses of gum in the youngest phellogen of Prunus Cerasus and thinks that 



1 Compare the second edition of this manual for older points of view. 



2 Beijerinck, M. W., and Rant, A. Wundreiz, Pai-asitimus and Gummifluss bei 

 den Amygdalaceen. Centralbl. f. Bakteriol. usw. 1905, XV, No. 12. Rant. A. Die 

 Gummosis der Amyg-dalaceen. Dissertation, Amstei'dam, 1906. 



3 Ruhland, W. Zur Physiologie der Gummibildung bei den Amyg-dalaceen. Ber. 

 d. Deutsch. Bot. Ges, 1907, Vol. XXV, p. 302. 



4 The gum exudation appears especially frequently in plums in wet years. As a 

 rule, it forms in little drops of gum as clear as water which come from wounds in the 

 fruit flesh made by insects. Often no insect injury can be recognized, and then the 

 places bearing the drops are usually harder and somewhat flattened. A considerable 

 accumulation of gum is found in the fruit itself beneath these flattened places. I 

 also found gummification of the pits of plums along the line of union of the halves, 

 so that under slight pressure the two fell apart. 



