406 



BOTANY OF THE LIVING PLANT 



hypha, which is believed to contain a digestive ferment (Fig. 342, i. ii.). 

 A ferment has been extracted from large cultures of a certain Botrytis, 

 and found to act upon cell-walls, causing them to swell. Such 

 swelling is a feature of the perforation by the invading hypha, which 

 first softens the cell-wall, and then seems to sink into the softened 

 mass, finally emerging on the other side (Fig. 342, iii.-viii.). This 

 power of perforation has been found in certain cases to depend upon 



the nutrition of the Fungus : for in- 

 stance, Sclerotinia sclerotiorum can only 

 penetrate living tissue after a period of 

 saprophytic nutrition. There is, how- 

 ever, another side to such questions in 

 the case of parasitic attack : viz. the 

 power of resistance of the victim, which 

 depends partly upon the thickness of 

 the protective walls, but probably also 

 on the presence or absence of inhibiting 

 substances. Thus fungal attack may 

 be regarded as a balance of physiological 

 powers between the invader and the 

 host. In fact, it stands on 'a footing 

 similar to that of mycorhiza in Phanero- 

 gamic Plants, or of conditions of sym- 

 biosis generally (see Chapter XL). 



It is along such lines that the explan- 

 ation 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 

 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. 413). 



