490 JOURNAL OF THE ROVAL HORTICULTURAL SOCIETY. 



" A different method of attack was therefore resolved upon, and in 

 the winter of 1909, after all the dead foliage had been removed as 

 usual, the ground was heavily dressed with slaked lime (the refuse 

 from an acetylene generator being used after allowing it to stand 

 exposed to air for about six weeks), the object in view being to counter- 

 act any soil acidity and to encourage the oxidation of organic 

 matter in the soil. Fungi, as a rule, are encouraged by an acid condition 

 of the soil, and the particular one in question is no exception. 



" The result surpassed our expectations, for the growth came away 

 clean and healthy in the next spring, and remained so through the 

 season, to the evident advantage of the plants. An occasional winter 

 dressing of Hme has been given, and extremely Httle of the fungus has 

 made its appearance since." 



The well-known authority on Iris, Mr. W. R. Dykes, writes me that 

 he is unable to give a very confident answer regarding the disease 

 because he has not kept accurate notes on the subject. " So 

 far, however, as my observation goes, the disease only occurs on 

 lime-loving species, and only when the soil is deficient in lime. At 

 any rate, since I took to dressing my Iris beds with superphosphate 

 in summer to check Pseudomonas, and chalk in winter to counteract 

 the acidity, I have practically been free from Heterosporium, but whether 

 the phosphate or the lime was really responsible I am unable to say. 

 I have never noticed the disease on the lime-hating Apogons, such 

 as the Sibirica group, and the Cahfornians. ... I should certainly be 

 incHned to agree that the disease spreads more rapidly in wet seasons, 

 possibly, I suppose, because the plants are then less healthy, for all 

 Pogoniris like a dry summer and autumn. I have not seen the disease 

 in the South of France in heavy soil, calcareous it is true, and very 

 dry in summer. I don't think heavy as opposed to Hght soil is favour- 

 able to the disease, for the general rule is that, given good drainage, 

 Pogoniris species are much happier in a heavy Hme soil than in sand." 



The extracts from Mr. Dykes' letter are very suggestive when read 

 in conjunction with Mr. Chittenden's note above. 



It certainly looks as if the presence or absence of sufficient Hme in the 



soil were a primary factor in determining whether the disease is present 



or not. What relation is there between calciphilous plants, which 



become attacked when there is insufficient lime, and calciphobe 



species, which are immune ? There is to be added the fact that 



a heavy dressing of Hme clears the plants of the disease. On the 



face of it, it suggests itself that the calcicoles are not strong enough 



physiologically to withstand the disease when they are grown in soil 



which has insufficient Hme. It is weH known that ah organisms in 



a weak physiological state are Hable to disease, and this of course holds 



in the case of fungi.* But with the few facts before us it is impossible 



to go further. It would be interesting to try whether the Apogons 



are really immune, and also whether a calcicolous species suppHed 



with sufficient lime can become diseased. 



* Cf. F. T. Brooks, "Observations on the Biology of Botrytis cinerea," Ann. 

 Bot. xxii. (1908), p. 482, 



