W. MPRAF. 237 



with small irron;ular brown markings and the cortex was slightly hrown. 

 This discoloration extended a little beyond the point of insertion of the leafless 

 side-branch whose tissues for 2 centimetres were slightly discoloured. Hypha^ 

 of Phytophthora were found in the discoloured tissues and copiously in the 

 medullary rays and in some of the larger vessels. The fungus was not found 

 on the surface of any of the branches. The shaded part and the crosses show 

 the limits within which hypha? were found, while the asterisks indicate the 

 regions where sections were taken in the search for hvphnp ; none, however, 

 were found there. The character and measurements of the hyphao proved 

 that this was the fimgus that is being investigated. On incubating pieces of 

 the discoloured tissue sporangia of PhytophfJinra were developed. One or two 

 of the leaves on the single branch that was shedding its leaves were secured. 

 They came ofE easily when the specimen was being cut showing that an absciss 

 layer had already been formed cutting off each from the branch. No dis- 

 coloration was found on the petioles or leaflets of these 'eaves, nor on any of 

 the leaves on the other branches. In this case the infection of the branch and 

 the falling of the leaves took place in the same season in a way very similar to 

 that of the former two cases in which the infection from one season caused 

 the wilting of the leaves at the beginning of the succeeding season. This 

 is another example of the fungus within a branch giving a stimulus that 

 causes the branch to shed its leaves. 



Thus when the lower part of a twig is invaded by the fungus, two things 

 may happen : -(1) In the early part of the season the leaves on the twig wilt 

 and fall off, and (2) in the monsoon they fall off being quite green or having 

 previously turned yellow, and this even though they are not themselves invaded 

 by the fungus. The presence of the fungus and of tyloses in the vessels suggests 

 that the upward flow of the sap is being checked, but they do not seem to be 

 present in sufficient quantity to fill the vasa to such an extent as to clog them 

 and directly retard the passage of the sap sufficiently to cause the leaves to 

 wilt and to fall. It seems more likely that the copious development of hyphae 

 in the medullary rays causes the cells of these rays to lose their pumping 

 function, and that accordingly the sap does not pass or passes in much 

 diminished quantity beyond the region of the stem infected by the fungus. 

 As already pointed out, most of the leaves that fall in the second leaf-fall 

 during the monsoon are directly infected, but a proportion that are not 

 infected fall for the cause just stated, and in the early part of the 

 growing season before the monsoon comes this also happens to a very 

 limited extent. 



