16 REACTION OF HOST TO PARASITIC ATTACK. 



absorption by haustoria results in a deformation and distortion 

 of attacked organs, which embraces even cells far distant from 

 the point of attack, yet without death following directly to 

 any cell. 



2. Absorption of cells or tissues by parasitic fungi The 

 total absorption of cells or tissues by parasitic fungi constitutes 

 a special form of cell-destruction. Cases of this kind occur 

 particularly amongst the Ustilagineae. Thus Urocystis violae so 

 stimulates the cells of Viola that they divide and produce a 

 delicate tissue, rich in protoplasm ; this nutritive tissue is used 

 up when spores are formed, but without any great detriment to 

 the host-plant. At the time of spore-formation of other Ustila- 

 gineae a great destruction of the host-tissues may, however, 

 take place ; this is especially marked in attacks of Ustilago 

 maydis, U. avenue, Tilletia tritici, on the ovaries of their 

 respective hosts, as well as in other cases to be considered 

 later. 



3. Killing of host-cells and tissues by fungi which excrete 

 ferments- The simplest case under this heading is presented 

 by species of Selerotinia studied by De Bary, e.g. Scl. sclerotiorum. 

 The mycelium of these, while still lying on the outer surface of 

 the host-plant, excretes a ferment which sinks through the mem- 

 branes into the cell-cavities, causing death to the protoplasm and 

 even destruction of whole tissues. 



A similar process may be assumed in the case of numerous 

 fungi with a mycelium which grows only in the intercellular 

 spaces, yet causes immediate death to any cell it may touch. 

 This is the case with many leaf-spot diseases, like those due to 

 Cercospora, Hysterium, etc. So also do the apices of rhizomorph- 

 strands km portions of the bast of living Conifers with which 

 they may come in contact. The rapid death of tissue following 

 the attpck of such deadly fungi as Phyto-phthora is probably 

 due not altogether to the deprivation of nutriment, but also to 

 the effects of a poisonous excretion. This, however, has not as 

 yet been satisfactorily ascertained. 



B. Killing of Organs or Whole Plants. 



A large number of fungi have a mycelium which never ex- 

 tends beyond a very short distance round the point of first 

 infection, and causes only local disease, frequently with no 



