VIII] 



UREDINALES 



215 



vegetative nucleus has replaced that of the no longer functional male 

 element. As already shown there is a strong presumption that this male 

 element was the spermatium and the fertile cell may then be regarded 

 as an oogonium and the young aecidium as a group or sorus of female 

 reproductive organs. 



In this connection Blackman has suggested a possible origin of the 

 sterile cell ; in Phragmidium violaceum he found it to be occasionally 

 elongated and pushed up between the cells of the epidermis so that it was 

 covered only by the cuticle (fig. 194); if in the past it broke through this 

 also, it would have formed an efficient trichogyne and may well have func- 

 tioned as such. 



In the related species Phragmidium speciosum, Christman, in 190.5, 

 described a similar development of sterile and reproductive cells, but in 

 this case the fertile cells become inclined one towards another in pairs and, 

 at the point of contact, the walls dissolve so that the protoplasts come into 

 relation, at first through a small pore, but later along the greater part of 

 their length. Binucleate cells are thus formed (fig. 195), conjugate division 



Fig. 194. Phragmidium violaceum Went. ; 

 caeoma; sterile cell pushing up between 

 epidermal cells of host, x 1300; after 

 Blackman. 



Fig. 195. Phragmidittm speciosum Fr. ; 

 fertile cells after conjugation ; aecidio- 

 spore mother-cell above ; after Christ- 

 man. 



takes place and aecidiospore mother-cells are cut off so that a single row 

 of aecidiospores is developed from each pair of gametes. Christman regards 

 the fertile cells as isogametes between which conjugation takes place, and 

 the sterile cells merely as buffers, of which the function is to assist in the 

 rupture of the epidermis. 



His observations on Phragmidium, speciosum were confirmed in 1906 by 

 Blackman and Fraser for Melampsora Rostrupi on Mercurialis (fig. 174); 

 these authors pointed out that the fusion of the fertile cells is a reduced 

 fertilization, strictly comparable to that in Ph. violaceum but achieved by the 

 union of female cells in pairs instead of by the entrance of a vegetative 

 nucleus into the female cell. This interpretation is confirmed by the fact 



