MICROSCOPIC GARDENING. 



disease," or hop and rose mildews. Still different must be our 

 procedure when dealing with " Dry-rot " and the various diseases 

 of trees, and so on. 



During the course of the investigations which have gradually 

 perfected our knowledge of microscopic gardening, another aspect 

 of the matter has slowly forced itself on our attention. 



Even the earliest exact observations on the infection of 

 plants by parasitic fungi raised the question as to the behaviour 

 of the host-plant. Can we regard a living leaf or root, &c, as a 

 mere passive soil on which the germinating parasite grows ; or 

 must we not rather assume that it plays a more active part in 

 the matter ? 



Long ago De Bary, impressed by the remarkable behaviour 

 of the infecting germ-tubes, hazarded the conjecture that the 

 contents of the cells of the plant attacked must probably re-act 

 in some peculiar manner to the invading organism, and some of 

 the most wonderful results of twenty-five years of microscopic 

 gardening have assured us that his conjecture was well founded. 



Curiously enough, this aspect of the question first came into 

 prominence during some observations on microscopic gardening 

 made by Pfeffer in an entirely different connection. He found 

 that when the zoospores of certain Saprolegnias congregate 

 round a piece of fly's leg, or bacteria round a bubble of air or a 

 piece of meat, or the spermatozoids round the mouth of an 

 archegonium of a moss or fern, their movements towards these 

 centres of aggregation were such as could only be explained on 

 the assumption that some attractive influence compelled them 

 towards the object they centred around, and he discovered in 

 each case that a definite chemical body exerted the attraction. 

 So complete was the proof, that Pfeffer could attract any of these 

 organisms out of a mixture into microscopic tubes of the par- 

 ticular chemical which exerts this curious attraction. 



Pfeffer's pupil, Miyoshi, then showed that fungus-hyphae 

 are subject to similar chemotactic influences. If a fungus is 

 allowed to grow in a microscopic garden-bed containing a mere 

 t race or none of the particular chemical found to attract it, and 

 another microscopic garden-bed at a little distance contains more 

 of this substance, the fungus leaves the former bed for the latter. 



Now the suggestive part of the matter conies in when we 

 learn that just these particular attractive chemicals are formed in 



