PLANT PHYSIOLOGY. 45 



In general it may be said that any physiological work of this 

 character is likel}' to yield results of direct value to the glass- 

 house grower, who is able to control the conditions of cultivation 

 of his crops to an extent that is impossible to the farmer. 



Resistance to Disease. 



We may now refer to some very important research work, 

 the object of which is to throw light on the complicated 

 question of immunity from disease. It is well known 

 that most disease fungi and bacteria show a preference 

 for certain plants, which they will attack and injure, while 

 other types of plants in the neighbourhood remain free from 

 infection. It is also known that in the case of certain diseases, 

 such as yellow rust of wheat and wart disease of potatoes, it 

 has been found possible to breed varieties of the plant which 

 are immune from the disease. One of the most important 

 benefits that the plant breeder can confer upon the farmer or 

 horticulturist is to produce for him a variety of plant which 

 will resist attack from a disease that causes serious losses. 

 Already much has been done in this direction by purely empirical 

 methods; but unfortunately it often happens that an immune 

 variety is not so good in other respects as a susceptible, and that 

 it is found difficult to combine immunity with other desirable 

 qualities such as high yield or first-rate flavour. This was so, 

 for example, in the case of wart disease of potatoes, where 

 until recently the immune varieties which were produced as the 

 result of prolonged experimental work were not equal in cropping 

 power to the susceptible sorts. If the plant breeder could be 

 told exactly what it is that makes one variety resistant to disease, 

 he would be in a much better position to reproduce that quality 

 than he is at present. Now, in a sense, he is working in the 

 dark; he must consider the plant as a whole, and find out, by 

 infection experiments, whether or not it is liable to attack. 

 With more definite knowledge of the nature of immunity he would 

 be able to confine his attention to the particular factors causing 

 immunity, and his sphere of usefulness would be greatly increased. 



In its essence the problem of fungoid disease is a physiological 

 one; it is concerned solely with the hfe processes of the fungus 

 and of the " host " plant on which the fungus becomes estab- 

 lished. It is also extremely complex. The infection of a 

 particular plant by a particular fungus follows upon a long series 

 of physiological actions on the part of the fungus, and simul- 

 taneous actions or reactions on the part of the plant. The 

 question is further complicated by the fact that the method of 

 attack is not the same in all cases. Sometimes the fungus enters 



