BOTANICAL SOCIETY OF CANADA. 
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composed the cells must suffer, and the morbific agency is at once apparent. But 
there is another point in which their action is not unimportant, viz., the power which 
fungi have of inserting themselves amongst the cells and tissues. Physiologists, and 
especially medical writers, overlook this fact, that a cell confined in a limited space, 
and at the same time undergoing development, must expand in some direction, and 
the force thus generated is almost incredible. Many of you have no doubt seen a 
strong wall pushed down by the growth of a tree ; that is, by the expansion of soft 
and otherwise yielding cells. But perhaps a more impressive fact is, that simple 
cellular fungi, growing under large stones, have raised them from their beds to the 
height of some inches, even when the stones were several hundred pounds in weight ; 
and yet so soft is the structure of the plant that it might be crushed between the 
finger and thumb. Here is a power not to be ignored when discussing the influ- 
ence of parasites. Let us see how it applies to the production of disease in animal 
tissues. Each individual cell, it must be borne in mind, possesses the same motor 
power ; it is only their combined action which yields great results such as the above. 
Suppose then a single tube inserted into the skin and impinging upon a nerve fila- 
ment, would you not expect that nerve to resent the intrusion ? Would it not do 
so if any other foreign body of the same size were introduced ? How much more 
then, if in addition to mere mechanical irritation, the cell proceeds to abstract or 
decompose the fluids. That it does this, which is indeed the essential function as 
a scavenger, we see in favus and ringworm, where, especially in the former, the 
odour produced by it is intolerably fetid and irritating. It is clear that what with 
the actual pressure of the outspreading fungus, and the irritating products which 
it engenders, there are strong prima facie grounds for believing that the fungus 
does actually produce disease. 
Then again if proof were wanting, observe the peculiar character of lichen 
annulatus^ fairy-rings in miniature, presenting all the characters that fairy-rings do, 
and showing clearly enough that the fungus and rings of inflammation proceed 
pari passu. 
The form of the disease will be determined by several minor conditions affect- 
ing the growth of the parasite ; these we have before mentioned as warmth and 
moisture, suitability of food and density of tissue, all of which influence the develop- 
ment of the plant ; thus we find in Lichen^ one form ; in Pityriasis^ another ; in 
^avus, a third, and so on ; the spread of the disease being co-equal with that of 
the plant, and the degree of passive resistance which the tissues offer to its inroads. 
It must be admitted here, as in the case of plants, that an unhealthy condition 
of the structures and fluids is necessary to the development of a parasite, for with- 
out these it would be incapable of establishing itself. The first attack would in a 
healthy body be at once resented, and the intruder repelled. 
I would remark before concluding, that those diseases which are probably 
