76 BOTANY 



the plants can be quite healthy, but if gardeners tried 

 to breed an entirely white race, it would die of mal- 

 nutrition. 



All the innumerable questions of nutrition come very 

 near the borders of the study of pathology, for an ill- 

 nourished individual, even if it lives, is much more 

 liable to disease than a healthy one. 



The great sources of infected disease for the plant 

 world, as for the animal, are the fungi and bacteria. 

 The higher plants are attacked by innumerable small 

 parasitic forms of fungi, some of which finally kill the 

 host. The study of the fungal diseases of plants is an 

 enormous one, for there are thousands of species of 

 infecting fungi, and in some cases they have most 

 complex life histories and pass through cycles of two 

 or three generations which inhabit different hosts. In 

 the study of human and animal disease many instances 

 are well known now of the parasite inhabiting several 

 hosts, for instance, man in one generation and the pig 

 or the mosquito in another. So it is with plants, and 

 the disease which, works havoc with the grain crops 

 goes into a new generation that inhabits the Barberry. 

 Often the w r ork of connecting the different generations 

 of the same disease is rendered excessively difficult by 

 the elusive and unexpected nature of the cycles ; and it 

 is only by the most careful breeding of the fungus pro- 

 ducing the disease and by experiment that the actual 

 data can be separated and the life history of the disease 

 established. We are, once more, back in the highly 

 equipped laboratory and studying details under the 

 microscope. 



The economic importance of plant pathology is self- 

 evident, for the crops we eat are often attacked by 

 disease, much of which modern science has learned to 



