I^O BOTANICAL GAZETTE. [June, 



nata develop spores but few of which show any signs of the, 

 digitate processes. 



Forming within the host as these spores do, they are un- 

 der constant pressure because of their own growth and the 

 resistance of the host tissues ; hence it is that we may expect 

 to find a large number of malformations. In all the sub-epi- 

 dermal species studied one-celled forms (figs. 12^*, 13 g) were 

 found aggregated in the same pustules with the regularly 

 formed teleutospores, but cross sections, vertical to the sur- 

 face of host, always found them to be located around the 

 borders of the sorus or in places in which an upward expan- 

 sion was not permitted. 



As I did not in any case observe mature spores of this 

 form in portions of the sorus in which there would be freedom 

 of expansion, and as such spores are often found in sori of P. 

 graminis and other eruptive species, which for some reason 

 have failed to tear away the epidermal covering, I take it 

 that these spores are simply dwarfed forms due, perhaps, to 

 a lack of nourishment and excessive pressure at the proper 

 time for the formation of the cross septum. According to 

 this view of formation the term " mesospore"- used either ac- 

 cording to Sorauer or Dietel is not applicable to these anom- 

 alies as found in the species studied (foot-note 1). 



As the number in which they appear in relation to the 

 perfect spores is exceedingly variable, not only upon differ- 

 ent hosts but in different pustules on the same" host, I deem 

 it more probable that their occurrence in these species is due 

 more essentially to local conditions of development than to any 

 hereditary tendency. 3 Certain it is that pressure within the 

 crowded sorus is capable of producing an almost unlimited 

 number of irregularities in the spore forms. Such forms as the 

 one seen at/", fig. 13, are always to be found in the borders 

 of the sori, the curvature being due to continued pressure 

 exerted by more internally forming spores. Furthermore, 

 the typical spores of a species are found to arise from the 

 central area of the spore-bed, the position in which the spores 



2 Winter— Rabenhorst's Kryptogamen Flora, Pilze, vol. i, p. 133. 



Sorauer— "Between the uredo and teleutospores one often observes intermediate 

 forms (mesospores) which are really to be considered as simple transition forms." Pflan" 

 zenkrankheiten, ed. 2, vol. ii, p. 213. 



Dietel—'* These spores (mesospores) are not medial between two other spore forms ; 

 but the specks in which they occur stand themselves intermediate between the two gen 

 era, Uromyces and Puccinia." Morphologie und Biologie der Urt-'dineen, p. f> ; Botanis- 

 ches Centralblatt, vol. xxxii, 1887. 



3 This is not given a m explanation of the production of the mesospores as found in 

 such sp^ies as P. vexam Farlow and P. xporoboli Arth., which I have not -tu<!#d. 



