Disease Resistance 



power of resisting the attack. Careful observation 

 will show that 



(1) Old leaves are very rarely attacked. 



(2) Some varieties are almost or quite immime 



from attack though their neighbours readily 

 fall victims. 



(3) A variety may be attacked in one garden and 



escape in another. 



Possibly the thick cuticle of the older leaves 

 preserves them from penetration by the fungus, but 

 what exactly renders one variety immune and 

 another prone to attack is not yet clear. Certainly 

 the glossy-leaved Wichuraiana hybrids are " cleaner " 

 than many hybrid teas, but it is probably not merely 

 a mechanical obstruction that preserves them. 

 Plants in a draught, or which have been subjected 

 to drought, or which are growing in a close, Ul- 

 ventilated spot, are usually the first to be attacked. 

 Is it because some substance is formed in them under 

 these conditions which attracts the fungus ? or is it 

 because some substance is not produced then which 

 under better conditions prevents the fungus attack ? 

 Is it because the fungus spores themselves require 

 to be subjected to a low temperature before they will 

 germinate ? or is it a combination of any or all of 

 these ? 



It is a remarkable, and probably significant thing, 

 that in the moist warm districts of the tropics 

 mildews are unknown, as they are, so far as I know, 



46 



