86 Resistance to Mildew m Ho2)s 



leaf or elsewhere. On the 16th dayS however, a leaf (at the 5th node 

 from the apex) of the shoot of OR 38 bore on. its upper surface one tiny, 

 powdery patch, of mildew ; the inoculated leaf as well as all the other 

 leaves (at the 11 nodes of the stem) of the plant were at this date and 

 subsequently quite free from mildew. There were no signs in this 

 plant of any weakness or abnormal growth, and a close examination of 

 the one leaf bearing the patch of mildew failed to show any difference 

 in its colour or any injury to the epidermal cells which might account 

 for this strictly local susceptibility. The patch of mildew measured 

 2 mm. X 1mm., and was powdery with conidia. These conidia were 

 removed with a sterilised scalpel, and placed in a film of water on a leaf 

 (at the 2nd node from the apex) of the same shoot of Oi^38. The 

 other leaf at the same node was similarly inoculated with conidia taken 

 from patches of mildew on another normally-susceptible hop-plant. In 

 neither case did any infection result 2. The little patch on the leaf of 

 OR 38 noted above did not increase in size, although it produced a fresh 

 crop of conidiophores, so that 14 days later it was again powdery. The 

 patch then soon began to die away, and 6 days later was dead. After 

 the disappearance of the mycelium, a minute patch of brown, dead 

 epidermal cells became visible, such as is often found at. the centre of a 

 flourishing patch of mildew. This plant of OR 38 grew vigorously 

 throughout the remainder of the season, but although continuously 

 exposed to inoculation by conidia from upwiards of a hundred mildew- 

 covered plants surrounding it, no trace of any further infection resulted. 



Here, again, as ill Eayper. 2 (noted above) there was certainly no 

 general breaking down of the immunity of the plant, but only a strictly 

 local susceptibility. Of the hypotheses advanced above (p. 85) to 

 account for this phenomenon, (2) would appear to be ruled out. 



If we summarise the results of the above experiments, we find that 

 the 1 inoculated leaf of OR 39 resisted infection ; of the 5 inoculated 

 leaves of OR 38, 4 resisted infection and 1 became feebly infected, while 

 a very small, strictly localised patch of mildew appeared from some 

 unknown source on one uninoculated leaf Of the 5 leaves inoculated 

 of three other seedlings of the wild hop from Italy all became fully 

 infected, and in the case of each of these seedlings most of the leaves 



1 The date was May 22, 1917. 



2 A young leaf on this shoot of OR 38 was pricked with a pin at two places, causing 

 respectively one and three holes, and then inoculated over the wounds with conidia taken 

 from patches of mildew on various hop-plants. No infection resulted. (In other cases 

 susceptibility to the attack of the "wrong " biologic form of a mildew has been induced by 

 this method: see Annals of Botany, Vol. xix. p. 125 (1905).) 



