6 DISEASES OF CULTIVATED PLANTS 



spots, and divide it into two halves as exactly as possible, and 

 place each half in a tumbler of water ; the two tumblers with 

 their half-plants are then placed in an ordinary room, side by 

 side, at a temperature of about 2o c C, and one is covered 

 close with a bell-jar, and the other left uncovered. In a short 

 time often a few hours -the covered leaves become black and 

 rotten with the disease ; whereas the uncovered one will go 

 on looking fresh for several days, though it also succumbs at 

 once if covered. The question arises whether the rapid 

 spread of the fungus and the rot it causes here are simply 

 owing to the increased supply of water, as the tissues become 

 turgid in the saturated atmosphere under the bell-jar; or 

 whether we have not here again, in addition, a case where the 

 diminished access of oxygen to the interior of the tissues of 

 the host results in the accumulation of organic acids and 

 other substances, which make the excessively turgid cells and 

 thin watery cell-walls more than usually easy prey to the 

 parasite.' 



This experiment, I consider, supports my contention that 

 disease caused by fungi is dependent on weather conditions ; 

 or to state the case more exactly, it depends on an excess of 

 moisture in the air and absence of sunshine; a 'muggy' or 

 'stuffy' condition, as it is termed in the country. As to 

 whether the disease is accelerated by moisture or by lack of 

 oxygen, although very important in itself, has no bearing on 

 the contention that atmospheric conditions alone can cause 

 an epidemic of potato disease, for even if it is proved that 

 lack of oxygen is the determining factor, such lack of oxygen 

 in the tissues of the potato is due to the presence of an excess 

 of moisture in the air. No one ever saw or even heard of an 

 epidemic of potato disease during a dry season. 



The following experiments, conducted at Kew, show still 

 more clearly the effect of weather conditions in accelerating 

 or retarding fungus diseases. Three potatoes showing rusty- 

 stains in the flesh, indicating the presence of the mycelium of 

 potato disease {Phytophthora wfesfans), were each cut into two 

 equal parts. Each half potato was planted separately in a plant 

 pot, the soil and manure used being the same for all, and 

 was sterilised by steam. Three of the pots were placed in a 

 house having a temperature ranging between 70 and 80 F., 

 and very often with moisture at saturation point. Each 

 pot was covered with a bell-jar. The remaining three pots 

 were placed in a house without any artificial heat, and having 



