544 RESEARCHES ON FUNGI 



emergence of a germ-tube is probably decided by some stimulus 

 coming from the epidermis of the host-plant or by the action of 

 light.^ Teleutospores, uredospores, and aecidiospores, which have 

 relatively thick walls, are usually provided with one or more pores. 

 Each spore produces a single germ-tube and the function of a pore is 

 to provide a point of easy exit for a germ-tube. Where a spore has 

 a single pore, the germ-tube emerges from that pore (Fig. 204, A, 

 p. 505) ; and, where a spore has several pores, the germ-tube emerges 

 from one of the pores.^ When one or more pores are present in a 

 spore, the germ-tube never emerges anywhere else than through 



a pore. 



The teleutospore in Uromyces and Puccinia, e.g. Uromyces Fabae, 

 Puccinia graminis (Fig. 204, A, p. 505), and P. Phragmitis (Fig. 222, 

 b, p. 546) invariably has one pore for each cell ; and this pore 

 always defines the position of origin of a basidium. Most of the 

 other genera, e.g. Gymnosporangium and Melampsora, have one 

 pore or none for each cell. In many species of Phragmidium each 

 cell has from two to three pores (Fig. 220, c). It is clear, therefore, 

 that, in the Uredineae generally, if we except Phragmidium which 

 has a loose teleutospore-sorus and many-celled teleutospores, the 

 rule is that each cell of a teleutospore has one pore only when a 

 pore is present at all. 



The uredospore, in all the higher Uredineae, has from 2 to 10 

 pores, and in most species from 3 to 6. Thus the number of pores 

 in each uredospore is : in Puccinia Vincae, 3 (Fig. 223, c, p. 547) ; 

 in Uromyces Fabae, 3 to 4 ; in Puccinia graminis and P. Phragmitis, 

 4 (Fig. 222, c, p. 546) ; in Hyalopsora Aspidiotus, 4 to 8 (Fig. 221) ; 

 and in Puccinia dispersa, 7 to 10.^ Out of many hundreds of species 

 included within the genera Uromyces and Puccinia there are only 



1 With the thin-walled poreless basidiospores of the Uredineae may be contrasted 

 the thick-walled monoporous basidiospores of certain of the chromosporous Hymeno- 

 mycetes, e.g. Coprinus sterquilinus. Here the pore is always terminal and from it 

 the germ-tube always emerges. 



2 Occasionally where a uredospore has several pores, as with certain pollen 

 grains having several pores, germ-tubes may emerge from two pores ; but only one 

 of the germ-tubes continues its development, while the other remains in a rudi- 

 mentary condition. 



3 W. B. Grove, The British Rust Fungi, Cambridge, 1913, pp. 97, 177, 251, 260, 



273, and 374. 



