46 AGRICULTURAL RESEARCH AND THE FARMER : 



through a wound in the plant, and can feed directly upon the 

 plant juices ; at other times the fungus enters through the outer 

 " skin " of the plant, and the resistance offered by this skin must 

 be overcome before the fungus is able to gain access to the food 

 material contained in the plant tissues. Even if this resistance 

 is broken down, the fungus may be unable to live on the juices 

 of some plants, and cannot cause an outbreak of disease. It is 

 not surprising therefore that the work which has been going on 

 at the Imperial College for the past few years has as yet only 

 touched the fringe, as it were, of this difficult problem. On the 

 other hand, most valuable preliminary results have been obtained,, 

 and there is good ground for expecting that further research 

 will be successful in providing definite information which will 

 be useful not only to the plant breeder, but also in suggesting 

 new methods of controlling fungoid diseases. 



It would be out of place in this pubhcation to refer to the 

 details of the methods that have been employed in tracing the 

 various stages in the penetration of the host plant by the minute 

 fungus parasite: The work is very difficult, and involves the 

 use of electrical apparatus as well as a considerable knowledge 

 of organic chemistry. It is necessary to study the action of 

 the fungus on numerous plants, some of which may for various 

 reasons be very unsatisfactory material for experimental work. 

 Further, the behaviour of different species of fungi may vary,. 

 so it has been necessary to examine several species, although 

 for the accumulation of comparative data the work must perforce 

 be done on one species only, and in this connection the fungus 

 Botrytis cinerea has been used. The experiments up-to-date 

 have established certain facts which are in a high degree important. 

 It has been found that in many case the " spores " (roughly 

 equivalent to seeds) — which are of course the normal agents of 

 infection — germinate in the moisture coating the outside of the- 

 plant, but are unable to pierce the outer skin (or epidermis) 

 unless they are sufficiently vigorous, and that the feeding^ 

 necessary to supply this vigour does not come solely from some- 

 thing within the " spores," but is exuded from the plant. Thus 

 we already see one reason why some plants are attacked and 

 others not. In the one case, the plant can exude through its 

 skin — perspire, so to speak — chemical substances which form a 

 food supply for the fungus spores, and enable them to develop 

 the degree of strength necessary for piercing the skin and entering 

 the tissues of the plant. In other cases such chemical substances 

 are not exuded, or it may even happen that substances arfr 

 exuded that are actually injurious to the fungus, so that it is 

 prevented from germinating and penetrating the skin. 



