200 



PROTOBASIDIOMYCETES 



[CH. 



Fig. 170. Uromyces Poae'R&h&n.; aecidium just before 

 the epidermis is broljen through, x 310; after Black- 

 man and Fraser. 



Pliragmitis. A corresponding discoloration takes place around the young 

 aecidia, and there is thus some suggestion that the spermatia, when functional, 

 were carried to their destination by insects. 



The aecidia occur in groups, usually on the abaxial side of the leaf ; in 



them the aecidiospores are 

 produced in basipetal rows 

 (fig. 170) alternating with 

 small, abortive, intercalary 

 cells, by the disintegration of 

 which they are set free. They 

 may be carried to consider- 

 able distances by the wind, 

 and there is evidence that 

 they are sometimes distri- 

 buted by means of insects or 

 of snails. The mature aecidio- 

 spore is usually subglobose 

 or polygonal in form, it is 

 enclosed in a thick wall per- 

 forated by several germ-pores, 

 and contains red, yellow or 

 orange pigment, and always 

 two nuclei. In germination 

 a hypha is put out which 

 enters the host plant through 

 one of the stomata and so 

 penetrates into the inter- 

 cellular spaces. 



The development of the 

 aecidium begins by the mass- 

 ing of hyphae either deep in 

 the tissues of the host {Gyin- 

 nosporangium clavariaefonne, 

 Puccinia Poarnin (Blackman and Fraser '06), Pitccinia Falcariae (Ditt- 

 schlag '10)), or directly below the epidermis (JPhragmidium violaceum (Black- 

 man '04), Uroviyces Poae (Blackman and Fraser '06) (fig. 171), Puccinia 

 Claytoniata (Fromme '14)) ; these hyphae give rise to a more or less regular 

 series of uninucleate cells. These are the fertile cells, but, before developing 

 further, each, at any rate in the relatively primitive forms (caeomata), may 

 cut off one or occasionally more terminal sterile cells which ultimately 

 degenerate. The fertile cells may unite laterally in pairs (fig. 172), so that 

 binucleate compound cells are formed ; they may similarly pair with the 



Fig. 17 



171. Uromyccs Poae Raben. ; young aecidium, 

 X 370; after Blackman and Fraser. 



