HOW PLANTS ARE INFECTED BY FUNGUS SPORES 13 



has come to the conclusion that the respective susceptibility 

 or immunity of species of Bromas against the attack of 

 species of Puccinia dispersa is not at all influenced by 

 structural characters. His conclusions are summarised as 

 follows : ' The capacity for infection, or for resistance to 

 infection, is independent of the anatomical structure of the 

 leaf, and must depend on some other internal factor or 

 factors in the plant.' The factor for insuring infection I 

 consider to be the presence of a chemotactic substance in 

 the plant, to which the germ-tubes of the parasite respond. 

 The factor for immunity, if it may be so stated, is the 

 absence of the necessary chemotactic body. 



Infection of plants by the spores of parasitic fungi occurs 

 chiefly during the night in a state of nature. The following 

 account of an experiment bearing on this point, which I 

 have previously recorded, may be repeated here. ' A batch 

 of vegetable marrow plants in my garden were badly attacked 

 by [a mildew] Sphaerotheca humuli. Twelve young leaves, 

 showing no trace of the disease, were selected for experi- 

 ment. Six leaves were protected during the day (6 a.m. to 

 6 p.m.) in paper bags, and left exposed daring the night. 

 Six other leaves were enclosed in bags during the night 

 (6 p.m. to 6 a.m.) and exposed during the day. This 

 arrangement was continued for a week ; those leaves that 

 had been exposed during the night only were white with 

 the mildew at the termination of the experiment, whereas 

 those leaves that had been exposed during the day only 

 were free from the disease with the exception of a few 

 very small patches on three of the leaves.' 



Some of the reasons why infection occurs during the night 

 chiefly, are as follows. The surface of leaves is covered with 

 a film of moisture, a condition necessary to enable the spores 

 to germinate on the surface of the leaf, previous to the 

 germ-tubes entering the tissues. Owing to retarded trans- 

 piration or loss of water, by the leaves, the cells become 

 more fully distended with liquid contents, a condition 

 favourable for infection. The starch formed during the day 

 becomes dissolved during darkness into soluble glucose, 

 and this along with other substances furnishes an excess of 

 food which practically saturated the leaves, and possibly the 

 necessary chemotactic substances are present in greater 

 abundance also. 



During a succession of very dull, damp days, conditions 



