1 889. J BOTANICAL GAZETTE. 1 43 



a simple sorus. This development is well shown in the 

 structure of the teleuto-pustule of P. anemones-virginianae, 

 the fusion of the stromal hyphae often not being so complete 

 as in other species (figs. 8 and 9). In this species the hyphae 

 are often seen in cross-section between the simple sori, show- 

 ing that the vertical position taken by the hyphae, as usually 

 seen, is in a manner an enforced one, consequent upon the 

 development of the sorus. 



A further proof that these hyphae are not different from 

 the ordinary filaments is found in the fact that when not under 

 pressure at the top they continue to elongate (fig. 8 b). The 

 much thickened places on the covering of the pustules in 

 this species are due to these same hyphae which have passed 

 through the epidermis, forming a fused mass, seen in section 

 fig. 10 c. In all the sub-epidermal species on glumaceous 

 plants the sori are found between the leaf-veins, becoming 

 confluent in long lines, while the hyphae which inclose the 

 simple sori fuse to form the intersorial stromata (fig. 2 c) 

 which in thick vertical sections of the compound sorus or 

 pustule give the appearance of the intermixing with the spores 

 of the so-called paraphyses of the descriptions. In vertical 

 longitudinal sections of the pustule, these intersorial stromata 

 are found to be formed across the space between the fibro- 

 vascular bundles in great regularity of position. Whether 

 these structures will be seen in vertical cross sections of the 

 rust pustule depends wholly as to whether or not the space 

 between the fibro-vascular" bundles is wide enough for the 

 development of two or more simple sori. From this it will 

 be seen that the ordinary rust pustule of the sub-epidermal 

 forms is not a simple structure as it appears from the exterior, 

 but is compounded of many simple sori : and that the structures 

 lying between owe their form and position merely to a crowd- 

 ing and fusing together of the hyphae which separate the sim- 

 ple sori and are not separate bodies, paraphyses, but simply 

 extensions of the spore-bed (stroma). That they generally 

 appear as seen at c fig. 6 is because an optical section can 

 not reveal the cellular structure. Because of the opacity of 

 the walls and the smallness of the cavities, sections which 

 are to make this structure plain must not have a thickness to 

 exceed 10 >>.. In cross section the intersorial stromata ap- 

 pear as strips of regular tissue forming a net-work between 

 the fibro-vascular bundles with which they are connected, 

 while the spores fill up the intervening spaces (fig. n c). 

 The only apparent difference between this fungal tissue and 



