SOME ACCOUNT OF POLYSTIGMA EUBRUM. 333 



become longer and grow round one another in dense spiral 

 convolutions. They are designated by Fisch ^^ secondary 

 spores/' but Frank considers that they are merely clinging 

 organs, " Haft-organe/' being very variable in form and inca- 

 pable of germination. When they are placed upon the cuticle 

 of a plum leaf they give off sac-like prolongations, which pene- 

 trate into the interior of the cells, and from which the myce- 

 lium of the stroma is again produced. 



The fallen leaves soon rot, except in the parts occupied by the 

 stroma of the Polystigma; these become brown and then 

 blackish, persisting even till the following July, and furnish- 

 ing up to that time a constant supply of fresh ascospores. As 

 they dry they curl up with the convex surface, on which the 

 openings of the perithecia are situated, directed outwards, and 

 are easily rolled along by the wind. Fresh infections of the 

 host-plants continue to originate up to the end of July. The 

 spores will not germinate in the perithecia, but when dis- 

 charged germinated in two hours at 18° C, and produced their 

 brown clinging organs in twenty-four hours. 



It will thus be seen that probably the whole life-history of 

 Polystigma rubrum is now known. The only point which 

 is left in doubt is the mode by which the ascospores are conveyed 

 from the ground to the young plum leaves. Frank had the 

 idea of invoking the aid of insects for this purpose, but, since 

 the spores are small, it would seem that the wind is capable 

 of doing all that is required. The infection of the host-plants 

 therefore takes place from tlie foliage of the preceding year. 

 But the collection and removal of the old leaves would not 

 avail to prevent a new outbreak if the mycelium hyberuates, 

 as that of so many Fungi does, in the woody tissue of the tree. 

 To settle this point Frank investigated carefully the twigs of 

 the affected plants, but could find no traces of mycelium, not 

 even in the petioles of leaves severely attacked by the fungus. 

 In fact, it could be seen that the mycehum ceased at the edges 

 of the stroma. The parasite is therefore annual, and infection 

 takes place only from the ascospores direct. The chief in- 



VOL. XXIV. NEW SEK. ^ 



