W. F. Bewlby 157 



number of the individuals grew quite healthily, it was observed that 

 in one spot all the plants died. These showed all the symptoms usually 

 associated with the disease caused by Phytophthora cryptogea, but 

 examination showed that no pathogenic organisms were present in the 

 diseased tissues. Further inquiries led to the knowledge that the area 

 containing the dead plants was that in which the cresylic acid had been 

 upset. 



The deleterious effect which ammonia has upon plant growth is 

 frequently observed, and in no place, perhaps, is the result so marked 

 as in the glass houses where high temperatures prevail. In the early 

 stages of the tomato crop examples can frequently be seen of "damping 

 off" effects produced through the rapid diffusion of ammonia from soil 

 which contains dung not sufficiently matured. 



The investigations to be described have been directed towards the 

 congeries of diseases caused by pathogenic fungi. 



Sources of Infection. 



At the outset one is faced by a number of possible sources of infection 

 all of which require careful investigation if complete control of the 

 disease is to be effected. The seeds themselves, the seed-boxes and soil, 

 the stages upon which the boxes rest, and the water used in cultivation 

 are all possible sources of infection. 



The Seeds. 



It is well known that the coverings of many seeds carry fungus spores 

 which germinate in the soil and destroy the young root as it emerges. 

 The tomato seed is especially suitable for carrying spores, for its testa, 

 being provided with long hairs, easily holds small particles. 



Apart from this, the method usually employed by the practical man 

 for the extraction of seeds from the fruits, increases the suspicion as to 

 the purity of the seed. Certainly the best fruits are chosen for the 

 purpose, but the subsequent treatment renders the seed liable to a great 

 deal of contamination. The fruits are first divided in halves and the 

 seeds with the mucilage which surrounds them are cut out into a pail. 

 In order to facilitate the removal of the mucilage, the whole mass is 

 allowed to ferment and soon becomes a mass of putrefaction, forming 

 a pabulum for all kinds of fungi and bacteria. This part of the process 

 is perhaps the most open to criticism because of the opportunity it offers 

 for the rapid growth of any organism, pathogenic or otherwise, that may 

 be carried to the fermenting matter. The length of time the mucilage 



Ann. Biol, vn 11 



