236 Transactions British Mycological Society. 



infection. In connection with the point last mentioned, it is a 

 well-known axiom in medical science that the danger of in- 

 fection increases with the magnitude of the dose of the disease, 

 so to speak, to which one is exposed; there is no doubt, for 

 instance, that even the healthiest man would succumb to tuber- 

 culosis if he inhaled an atmosphere saturated with the tubercle 

 bacillus. With plant diseases the same principles are only just 

 beginning to be recognised. The means by which the germs of 

 these diseases are brought into contact with their hosts are often 

 different from those of animal diseases, but they are none the 

 less sure. It is now known that many infectious plant diseases 

 are chiefly spread by insect agency, and occasionally by birds. 

 There is a widespread group of plant diseases of great economic 

 importance, known as mosaic diseases, which are characterised 

 first by a mottling of the leaves with yellow spots and then by 

 general degeneration. No organisms have yet been isolated 

 which can be considered the cause of these diseases, but if the 

 sap of an affected plant is inserted into a healthy one, the latter 

 soon becomes diseased. Such diseases and others of a similar 

 nature are known as viruses. They are of common occurrence 

 in animals and are often supposed to be due to organisms of 

 ultra-microscopic size. Foot and mouth disease in animals and 

 the mosaic diseases of tobacco and potatoes are maladies of this 

 nature. Mosaic disease of tobacco is one of the chief factors 

 limiting the growth of the finest kinds of tobacco in those regions 

 of the tropics, such as Sumatra, otherwise favourable for their 

 growth. It has been shown recently that these mosaic diseases 

 are largely spread by means of insects, especially by aphides, 

 which, having visited diseased plants, puncture healthy ones, 

 and thereby insert the virus. Apart from mosaic diseases, in- 

 sects play an important role in the dissemination of plant 

 maladies caused by known parasitic organisms, whether bacteria 

 or fungi. The insect in sucking or biting the tissues of a diseased 

 plant inevitably comes in contact with the germs of the parasite, 

 some of which adhere while it passes to another plant, which 

 may thus become infected. In fact, so intimate is the incidence 

 of certain insect and fungus pests, that widespread opinions 

 prevail amongst cultivators that fungi inevitably follow par- 

 ticular insects. One of the commonest diseases of apple trees is 

 "canker," caused by the fungus, N ectria galligena , which some- 

 times follows in the wake of woolly aphis, one of the most serious 

 insect pests of apple trees. This sequence is probably assisted 

 in two ways, first by the insect carrying the spores of the fungus 

 entangled in its woolly appendages, and secondly because the 

 insect punctures the bark, thereby creating wounds by which 

 the fungus can invade the branches. Again, in the rubber plan- 



