160 The Fungus of the Haivthom. 



agreeing with Greville's description. The circukr spots on the leaves, 

 Nos. 1 and 2, indicate the general points of growth of this fungus. 

 I find it frequently on the leaf-ribs and terminal points of the 

 leaves, and very often dispersed over the smooth parts of the leaf; 

 sometimes, although rarely, the peridia are on the upper surface of 

 the leaves. 3 represents the peculiar formation of their structure, 

 which resembles network. At the juncture of the leaf (see 4) the 

 cells of the peridia are nearly round ; at 5 oblong. From 3 to 4 

 the cellular structure is of a light Vandyke brown; at 5 a pale 

 yellow. I am aware that the structure of the peridia, as described 

 by me, varies in some respects from that by Greville and others, 

 which shows the importance of photographing so minute objects. 

 I have presented sections of these for future use. 6 represents the 

 appearance of the peridia as seen by the naked eye ; 7 their general 

 arrangement and their groupings on the leaves ; 8 three cells, 

 showing the parts of which the peridia are composed when mag- 

 nified about 125 diameters ; 9 the spores contained in the bottom of 

 the peridia magnified 125 diameters; 10 represents the openings or 

 meshes ; 1 1 a leaf of a variety of G. oxyacantha. On one of its 

 lobes, at A, is represented a cluster of peridia of Roestelia lacerata. 



14 represents the peridia of a species of Mcidium as they appear to 

 the naked eye, heretofore undescribed as a parasite on the haw- 

 thorn, and may have been confounded with that on the leaves ; 



15 a very highly magnified view of one of them, the cells of 

 which are magnified 125 diameters; 16 one of the cells some- 

 what more highly magnified. They are not always regular in 

 construction, although generally of the form given. They separate 

 easily from one another by slight friction, 17 represents spores of 

 an orange colour, with which the peridia abound, and which consist 

 of at least three parts : first, a transparent outward cell, which 

 contains an orange colouring-matter, within which may be seen 

 germinal matter in the form of dark spots. The spores are mag- 

 nified 125 diameters. 



All standard works on mycology represent Roestelia lacerata as 

 the only fungus of the order Mcidiacei that attacks hawthorn 

 plants ; but judging from my investigations, it holds a secondary 

 place. So conspicuous are the species of the two genera on them, 

 the one on the leaves, the other on the branches and fruit, that 

 the naked eye can distinguish the difference. That on the leaves 

 appears of a brown colour. Owing to the transparency of the cells 

 of the peridia, the brown colouring-matter of the protospores being 

 seen through them, while that on the fruit and branches appears of 

 a rich orange, owing to the colour of their protospores. Although 

 of the same order they differ in genus and species. 



It is of much importance to ascertain as far as possible the con- 

 ditions of growth favourable or unfavourable to this order. Its 



