404 MINNESOTA BOTANICAL STUDIES 



Riibiis sp. might be accounted for by the rupture of more or less ex- 

 tensive areas of the lower epidermis of the leaf due to the "Caeoma" 

 type of sorus produced by Gymnoconia interstitialis. This would 

 obviously accelerate transpiration from the underlying mesophylHc 

 cells. But here also possibly other factors connected with the dis- 

 eased condition may operate to cause increased transpiration. Per- 

 haps deleterious substances are thrown ofT by fungi which mate- 

 rially alter the transpiration rate. 



Reed' has shown that small amounts of tannic acid and pyro- 

 gallol accelerate transpiration. He also pointed out that substances 

 like oxalic and acetic acid, which are found as such in plants, when 

 in small amounts increased transpiration. Similarly Burgerstein,- 

 working with solutions of camphor, found that solutions containing 

 one part of camphor per thousand had an accelerating effect upon 

 the transpiration of most plants investigated. Excised shoots which 

 were previously allowed to become wilted revived more quickly 

 when placed in camphor water than when placed in distilled water. 

 By weighing the vessels of water in the two cases it was shown that 

 transpiration from the shoots went on more rapidly in camphor 

 water than in distilled water. That camphor was absorbed by the 

 excised shoots was shown by their pathological condition and death 

 prior to the appearance of any such conditions in the parallel series 

 in distilled water. 



This may properly be regarded as an example of transpiration 

 under pathological conditions and it may be that deleterious sub- 

 stances thrown off by fungi act similarly to the camphor. However, 

 only after careful biochemical investigations concerning the activi- 

 ties of invading hyphae and invaded host tissue are made can this 

 question be definitely settled. 



Whether due solely to the ruptured epidermis or in part to other 

 causes acting in conjunction with this, it is certain that rusts in- 

 crease transpiration to a marked degree even when infection is light, 

 while a moderate infection may more than double the normal rate. 

 Livingston^ has shown that for w^heat grown under ordinary trans- 

 piring conditions, the amount of transpiration is a rather reliable 

 index of growth, when the latter is measured by the increase in 

 green weight of the above-ground parts. However, it seems clear 

 that the transpiration rate might under diseased conditions of the 

 plant become so excessive as to be highly detrimental to it in carry- 

 ing on its normal activities. 



