136 diseasp: in plants. 



actual absorption from the external cells. Here 

 the fungus is a parasitic epiphyte. 



A stage further is attained in those fungi which 

 enter the stomata and live in the intercellular 

 spaces e.g. many Uredineae and PhytopJithora 

 and many such intercellular endophytes increase 

 their attack on the cells by piercing their walls 

 with minute {Cystopus) or large and branched 

 {Peronospord) haustoria, or even eventually pierce 

 the cells and traverse them bodily {Pythiuin). In 

 all these cases it is clear that conflicts must 

 occur between poison and antidote, acid and 

 alkali, attractive and repellent substances, enzyme 

 and enzyme, etc., as was hinted at above ; and 

 the same must take place when the parasite is 

 endophytic and intracellular from the first, as 

 in Chytridiaceae, etc., the zoospores of which 

 pierce the outer cell-walls and forthwith grow 

 into the cells. There are also fungi which, while 

 able to pierce the outer cell-walls, and grow 

 forward in the thickness of the wall itself, cannot 

 enter the living cells themselves ^.^. Botrytis. 

 In the example mentioned, the fungus excretes a 

 poison, oxalic acid, which soaks into and kills the 

 cells next its point of attack : into these dead cells 

 it then extends, and, invigorated by feeding on 

 them, extends into other cell-walls and excretes 

 more poison, and so on. 



On the basis of the foregoing it seems possible 

 to sketch a general view of the nature of parasitism. 

 In order that a fungus may enter the cells it must 

 be able to overcome not only the resistance of the 



