i| PARASITISM 15 



The relations of the parasitic fungus to its host are extremely various; 

 some,especiallycertain unicellular parasites (<9^/<#«w, Woronina)go through 



tlieir whole development inside the cells of the host, some live wholly in the 

 intercellular spaces (Gnomonia) or under the cuticle (Exoascus, Taphrind) 

 and obtain nutriment by osmosis; others possess an intercellular mycelium 

 from which short branches are sent into the living cells of the host; these 

 branches may become specialized to form haustoria of limited growth and 

 more or less definite form (Cystcpus, Ustilago, etc.). In all the above cases 

 the parasite is inside the host and may be described as endophytic. 



In a certain number of forms the development of the parasite is external, 

 and may be described as epiphytic. This is the case in many mildews 

 (Erysiphaceae) which obtain their food supply by sending haustoria into 

 the epidermal cells of the host; it is also the case in the Laboulbeniales 

 where food is absorbed in many species through the unbroken membranes 

 of the host, and where the parasite probably causes a minimum of damage 

 and inconvenience. 



The influence on the parasite of its method of life is already shown at a 

 very early stage in development. Thus the conidia or zoosporangia of 

 Cystopus camiidus germinate better at low temperatures than high; their 

 minimum is near zero, their maximum about 25 C. and their most favourable 

 temperature, under ordinary circumstances, io°C. The spread of the fungus 

 by zoospores depends on the presence of water on the foliage of the host, 

 a fall of temperature leads to the deposition of dew, thus providing the con- 

 dition for the activity of the zoospores, and at the same time serves as a 

 stimulus to their development. 



In contrast to the above the uredospores of Puccinia dispersa, which give 

 rise to a germ-tube directly, germinate between IO and 27 C. but most 

 readily at about 20° C. ; the high temperatures appropriate to the germina- 

 tion of the spores of coprophilous fungi may also be recalled in this connection. 



The relation between the parasite and its host may be strictly localized, 

 as in the Laboulbeniales and in those Archimycetes which enter a single 

 cell and complete their development within it; or the parasite may spread 

 far beyond the point of infection, extending through or over the existing 

 parts of the host and keeping pace with its growth when the new tissues are 

 developed; in this way the mycelium of certain of the Ustilaginales is found 

 \ ear after year in the tissues of the herbaceous host, dying down when it 

 prepares for the winter, and growing with its growth; perennial mycelia are 

 not uncommon in Hxoascaceae.Uredinales and other forms which infect trees. 



The parasite may modify the host tissues by its invasion, chiefly in the 

 direction of abnormal growth < >r hypertrophy. The simplest instance of such 

 an effect is the enlargement of a single cell due to the entrance oi the para- 

 site; this is found in the infection of various algae and fungi by Olpidiitm, 



