102 MASSACHUSETTS HORTICULTURAL SOCIETV. 



separated hosts would, seem to indicate that there was a tendency 

 here to the development of forms which are characteristic on 

 different hosts, though these are not sufficiently distinct mor- 

 phologically for mycologists now to consider them different 

 species. A vivid imagination might suggest that in times past 

 perhaps the potato fungus was not so very different from the 

 Fhytophthora ractoimm, or, to speak more nearly to the point, 

 that the two were merged in one as a quite widely variable 

 species, but that they now have become quite well separated. If 

 this were the case it might explain the total absence, so far 

 as we know, of the sexual stage in the Phytophthora infestans, 

 i.e., the conidial stage might have become so different on certain of 

 the hosts, structurally as to produce a different and characteris- 

 tic form, and biologically as to hinder it from developing the 

 sexual stage. We must not forget, hoAvever, that the absence of 

 the sexual stage might be given a different interpretation, and 

 also that it may even now occur on some hosts which we have 

 not yet examined, or under conditions Avhich we have not yet 

 discovered. 



The white rust, Cystopus Candidas, which attacks many of the 

 species of the Cruciferse, might also be mentioned as an instance 

 of wide range in parasitism within a single family. The well- 

 known Puccinia graminis has a very wide range on the grasses 

 and grains, as compared with many other species of the Ure- 

 dineae found on the Gramineae. Other species might be men- 

 tioned, but this number serves well to show how omnivorous 

 some species are in their tendencies as compared Avith many 

 others. 



Tendency toward a Narroav Range in Parasitism. — 

 There is, on the other hand, a tendency Avith some species to a 

 restricted range in parasitism, so that so far as Ave knoAV a single 

 species is confined to but fcAV host species, or even to but one 

 host species. This is Avell illustrated by the common Fusicladlum 

 dendriticum and F. pirinuin. The former, the apple scab, is one 

 of the most common of the fungus diseases of the apple tree, 

 especially during wet seasons, affecting the leaves and fruit, and 

 causing dark spots by the groAvth of the threads and spores on 

 the surface. The Fusicladiuni pirlniim, the pear scab, attacks 

 the pear tree in a similar Avay, and so nearly alike are the tAvo 

 fungi tliat there has been some question among certain mycolo- 



