RESEARCH IN PHYTOPATHOLOGY 207 



of plants subjected to a damp atmosphere or other abnormal 

 conditions. 



Pasteur has determined that bacteria are not present in the 

 normal healthy animal tissues. This view is also generally 

 held with regard to vegetable tissues, but some more con- 

 clusive experiments are needed to decide this point in the case 

 of plants. 



In many physiological experiments connected with plants 

 the existence of bacteria, both on the external surface and 

 possibly in the intercellular spaces, is ignored but the action 

 of the various micro-organisms present must have contributed 

 in some measure to the effects recorded. That a neglect ot 

 such considerations may lead to serious misconception is ex- 

 emplified by the observations of Stoklasa. The generally 

 accepted view that an injured plant organism breathes more 

 actively than an uninjured one is shown by him to be incorrect. 

 The experiments by Stich which claimed to demonstrate this 

 point were not conducted under conditions free from bacteria 

 and when repeated by Stoklasa under sterile conditions were 

 found to give opposite results. Under proper precautions the 

 respiratory activity of injured cells proved to be less than that 

 of uninjured tissue and the increased production of carbon 

 dioxide at a wound was traced to the activity of the bacteria 

 living upon the injured cells. 



The external conditions to which any plant is exposed have 

 an important bearing upon its general health and render it more 

 resistant or more susceptible to parasitic attack. Phytophthora 

 infestans may be cited as a familiar instance. It is generally 

 recognised that light, and the temperature and vapour pressure 

 of the air, influence in a marked degree the destructive action 

 of this fungusi and presumably also of other fungoid and 

 bacterial parasites. Again the temperature, air and moisture- 

 content of the soil, and the nature of its food constituents, are 

 all forces necessarily affecting the general vigour of any host- 

 plant. There is considerable evidence that susceptibility to 

 disease is influenced by manurial treatment and that abundant 

 fertilisation, especially with nitrogenous manures, renders the 

 host less resistant to microbial invasion. Laurent has shown 

 how the susceptibity of a given variety of potato was related 

 to the manurial treatment under which it was cultivated. Thus 

 the variety Simpson, when grown in a soil manured with 



