154 DISEASE IX I'LAXTS. 



of an epidemic consisting in events which favour 

 the reproduction and spread of a fungus more 

 than they do the well-being of the host. 



As a third illustration I will take the case of an 

 insect epidemic. In 1863 a disease was observed 

 on vines in the South of France which frightened 

 the growers as they realised its destructive effects : 

 the roots decayed and the leaves turned yellow and 

 died before the grapes ripened, and such vines threw 

 out fewer and feebler shoots the following year, and 

 often none at all afterwards. In 1865 the disease 

 was evidently becoming epidemic near Bordeaux, 

 and in 1868 it was shown to be due to an insect. 

 Phylloxera, the female of which lays its eggs on 

 the roots, where they hatch. The louse-like off- 

 spring sticks its proboscis into the tissues as far as 

 the central cylinder. The irritated pericycle and 

 cortex then grow and form nodules of soft juic\- 

 root-tissue at which the insect continues to suck. 

 Rapid reproduction results in the majority of the 

 young rootlets being thus attacked, and since the\- 

 cannot form their normal periderm and harden off 

 properly they rot, and admit fungi and other evils, 

 in consequence of which the vine suffers also in 

 the parts above ground. 



Evidence that the general damage is due to the 

 diminished root-action is found in the peculiarly 

 dry poor wood formed in the " canes " of diseased 

 plants. 



By 1877 the epidemic had spread to the 

 northern limits of the French vineyards, and by 

 1888 half the vines in the country were attacked, 



