114 MASSACHUSETTS HORTICULTURAL SOCIETY. 



what occurs in the case of the bearing of some fruit trees, where 

 a tree Avhioh has been permitted to bear very lieavily one season 

 will usually bear but little fruit the next year, and so a sort of 

 alternation sets in, fruiting years being followed by years of 

 scanty fruit. It has seemed to me at times that the fluctuation 

 which sometimes marks the appearance one year of fungi in 

 great numbers, and then a scanty appearance the next year, 

 might find an explanation in a sort of periodicity. Climatic con- 

 ditions undoubtedly play a most important part in the period- 

 icity of many of the great fungus attacks, especially of those 

 fungi which are dependent on a new infection of the host from 

 year to year for their propagation. The perennial species of 

 leaf curl and plum pocket sometimes are very abundant one 

 season, and then the next season are comparatively rare. Tavo 

 years ago the specimens were very plentiful, and the following 

 year I made preparations for a harvest of the species in an early 

 stage of development, but it was almost impossible to obtain any 

 material. It seems strange that climatic conditions should be 

 solely responsible for the great abundance one year and the 

 scanty appearance the following year. Can it be that the myce- 

 lium one year largely spends itself in the production of the fruit- 

 ing form so that there is comparatively little mycelium left in 

 the buds of the tree or young tissue, and that the following year 

 the energy of the fungus is largely devoted to the replenishing 

 or building up again of a stock of the vegetative portion, so 

 that with the opening of the buds the next succeeding year it can 

 advance to the fruiting condition rapidly and abundantly ? 



Variation in Form. — In many fungi there is a tendency to 

 a great variation in form and habit in what is recognized as a 

 single species. This tendency to variation in form in the case of 

 certain species makes it exceedingly difficult in some other cases 

 to determine just the limits of the species. Certain species of 

 the Uredinea3, as Piurinia heterospora, and F. pruni-sjiinosa, 

 vary either in the number of cells in the spore or in the marking 

 of the spore. In the form genus Cercospora there is very great 

 variation in the length of the conidia, even in a single species, so 

 that there is some uncertainty at times as to whether this varia- 

 tion is due only to meteorological conditions or to a specific 

 difference in the fungus in question. Again, in the group of 

 fungi known as the i)Owdery mildews, there is the species 



