14 INTRODUCTION [CH. 



this colour change spreads through the mesophyll in advance of the fungal 

 hyphae. The hyphae, in fact, have been shown to secrete an enzyme which 

 both disintegrates the walls of the host cells and causes the death of the 

 protoplasts ; it is however quite unable to affect the cuticle, the penetration 

 of the outer walls of the epidermis being mechanical; a corresponding 

 mechanism has been demonstrated in Colletotrichnm Lindemuthianum and it 

 may be inferred that the same occurs in other less fully investigated cases. 



Penetration of the cuticle is,however,by no means a necessary preliminary 

 to parasitism, whether obligate or facultative, for the hyphae of many fungi 

 enter the host through the stomata, and in others, the so-called wound 

 parasites, infection only takes place where a previous injury has exposed 

 the internal tissues. 



It is a curious point, and one deserving further investigation, that while 

 the germ-tubes from certain fungal spores always make use of the stomata 

 as a means of entrance, certain others completely ignore them, even though 

 the germinating spore happens to lie close to a stoma; it is possible that in 

 the former case a hydrotropic or negatively phototropic stimulus and in the 

 latter a contact stimulus alone or in addition is operative. In Botrytis cinerea 

 hyphae sometimes grow into an already seriously infected leaf by way of 

 the stomata; this may be related to the fact that the stomatal space becomes 

 charged with fluid due to the breaking down of the host cells. 



A positively chemotropic reaction has often been suggested (Miyoshi) 

 as explaining the entrance of the germ-tube into the leaf, but recent in- 

 vestigation (Fulton, Robinson, Graves) show that positively chemotropic 

 sensitiveness is weak or absent in all the hyphae studied. 



In Botrytis the mycelium commonly enters the cells of the host, but in 

 other fungi it may develop wholly in the intercellular spaces, killing those 

 cells with which it comes in contact and benefiting by the food materials 

 that diffuse out through the dead protoplast; such forms may be described 

 as saprophytic in the same sense as Botrytis. 



As might be expected fungi whose attack is mainly directed to the 

 elements of the wood flourish equally well on living and on dead tissue; the 

 harm that they do to their host largely depends on the fact that they cut 

 off" the water supply to the regions beyond the infected area. This is the case 

 with Nectria cinnabarina, and the fact that in this species the ascocarps are 

 produced on dead tissue emphasizes its saprophytic or hemi-saprophytic 

 character. We have here a case where the survival of the host is of no ad- 

 vantage to the attacking fungus. 



Obligate Parasites. In the case of obligate parasitism, on the other 

 hand, it is evident that the death of the host involves the death of the fungus, 

 and it is to the interest of the parasite that death should be postponed, at 

 least until the latter has itself made provision for reproduction. 



