50 DISEASES OF TREES 



epidermis, the endophytes must send either their germ-tubes, 

 arising from spores germinating externally, or else their mycelia 

 into the interior of the plant. 



According to the mode of attack, we may divide parasites 

 into two main groups. The first comprises those which have the 

 power to attack uninjured plants ; the second, those which can 

 effect an entrance only through a wound. It is those belonging 

 to the latter group which are accountable for infectious wound- 

 diseases. The former are partly confined to the very early stages 

 of development of the plant, or of the shoots, leaves, or roots ; in 

 rarer instances they also force their germ-tube into the stomata 

 and lenticels of more mature leaves and shoots. It is only when 

 the mycelial growths are very vigorous, like those of Agaricus 

 melleus and Trametes radiciperda, that they are able, by entering 

 between the bark-scales of the root and forcing them apart, to 

 bore even into cortical tissues covered with corky layers. 



The mode of attack of Rosellinia quercina affords one of the 

 most interesting examples of this kind. The main root of the 

 young oak is protected against external attack by a corky mantle 

 of considerable firmness. The mycelium of Rosellinia is con- 

 sequently able to get at the interior only by first killing the fine 

 lateral roots, and as these traverse the corky layer the hyphae 

 form breaches in the protective covering. At the points where 

 the lateral roots pierce the corky mantle the mycelium develops 

 fleshy tubercles, which then send one or more processes through 

 the breach into the interior of the root. It is only some time 

 afterwards that the destructive filiform mycelium is formed at 

 the apex of these processes. 



Wounds admitting of the entrance of parasites into the interior 

 of trees arise in many ways. Reference need only be made here 

 to such agencies as animals, man, hail, wind, snow, &c. 



The effects produced by parasites on the tissues of the host- 

 plant can be explained only by assuming that in each species of 

 fungus a peculiar enzyme (ferment) is produced in its protoplasm, 

 which by being excreted through the hyphae is communicated 

 to the adjoining cells.* 



Very often the mycelium vegetates in living parenchymatous 



* [That such enzymes are really formed and excreted by the protoplasm has 

 been proved. For instance, Botrytis excretes an enzyme capable of dis- 

 solving cellulose see Annals of 'Botany , 1888, "A Lily-disease." ED.] 



