396 



BOTANY OF THE LIVING PLANT 



It is along such lines that the explanation must be sought for the 

 condition known as " epidemic," where by a sudden outburst a disease 

 becomes prevalent. Examples have been seen in the Irish Potato 

 Famine, the Coffee Disease of Ceylon, or the Lily Disease which in 

 1888 made the cultivation of Lilies in the Thames Valley a failure. 

 In such cases the disease is not necessarily a new one. The novelty 

 lies in the success of the invader. It appears to be due to a change 

 of balance between attack and resistance. That balance may be 

 affected either by physiological strengthening of the parasite, or by 

 weakening of the host. Sometimes the same circumstances may 

 affect both. In the Lily Disease and the Potato Disease a cold wet 



Fig. 295. 



Portion of the root of a Crucifer malformed owing to the presence of Plasmodiophora. 

 (After Woronin ; from Marshall Ward.) 



season, while it favours the fungus, produces a thin-walled, watery 

 host, readily susceptible to attack. A similar epidemic of " damping 

 off " by Pythium may at any time be induced by cultivation of Cress 

 overcrowded, in moisture and heat (see p. 400). 



The effect of the parasitic invasion may be the death of the host, 

 where vital parts are destroyed, as in attacks by Pythium, or Armillaria 

 mellea (Fig. 292). But in many cases the attack is tolerated by the 

 host, with only partial injury. It is often the leaf, or only certain 

 tissues of the leaf, which are attacked, the result being a loss of 

 efficiency by the host while the parasite gains access to the sources 

 of supply. The host may even be stimulated to greater action, with 

 the effect of swelling and extra divisions of its cells. The result 

 may be various malformations, such as are seen in the familiar leaf-curl 

 of Peaches or the swollen patches of Cluster-Cups (Fig. 337, p. 437)- 



