22 COMPARATIVE MORPHOLOGY OF FUNGI 



the nuclei, so that the prosorus is divided into 5 to 7, seldom into 9 por- 

 tions. These portions are surrounded by hyaline membranes; they are 

 the future zoosporangia in which, by further divisions, the number of 

 nuclei increases to as many as 300 (Fig. 11, 12). 



At germination the rosette, along with the whole sorus, is pressed to 

 the upper surface by the swelling of the tumor cells; similarly, by pressure 

 on all sides, the sporangia are forced out of the sorus and the disorganised 

 host cell, into the open (Fig. 11, 13) and liberate their numerous zoospores 

 through a narrow slit. 



Zoospores from over-ripe sporangia behave as planogametes. A zoo- 

 spore swims to another which has come to rest and fuses with it (Fig. 

 11, 13 to 15). It is improbable that zoospores from the same sporangium 

 copulate, although zoospores from different sporangia of the same sorus 

 may. 



The zygote, like a zoospore, penetrates the epidermal cell of the host 

 (Fig. 11, 16 and 17); under its influence the latter divides repeatedly so 

 that daughter cells and parasite are pushed several layers of cells deep 

 into the host tissue (Fig. 11, 18). 



While the haploid zoospores cause only a hypertrophy of the host cell 

 and the sori arising from them cause a hyperplasia of the neighboring 

 epidermal cells, the diploid zygote or young hypnospore causes hyper- 

 plasia of the host so that the resting spore is pushed deeper into the host 

 tissue, but the neighboring epidermal cells remain undivided. Here is 

 an example of the different specific effect of the haplont and diplont on 

 the host, such as we shall find in the Uredinales. 



The diploid nucleus extrudes chromatin at least thrice ; a true meiosis 

 has not yet been reported. The young hypnospore surrounds itself 

 with a double wall; a third layer is laid down by the protoplasm of the 

 host cell. After a long resting period, usually the next spring, numerous 

 zoospores are formed by repeated nuclear division (Fig. 11, 19); because 

 of the swelling of the innermost wall layer, the hypnospore bursts open 

 and the zoospores are set free in their sorus (Fig. 11, 20). 



Closely related to this form is S. aureum which lives on more than 

 100 host plants in very diverse families (Rytz, 1907). A much-enlarged 

 epidermal cell serves as host cell for the fungus. The neighboring cells, 

 especially the epidermal cells, enlarge and occasionally divide to produce 

 a hemispherical growth which often raises the host cell above the plane 

 of the epidermis. The top of the host cell (i.e., the top of the wart) lies 

 in a slight depression. According to observations in the natural habitats, 

 the several strains of fungi formed because of different conditions of 

 living and plant associations, differ on principal and auxiliary hosts, for 

 the principal host may be comparatively regularly infected and the 

 auxiliary hosts under definite, but still unknown conditions. Thus for the 

 Lysimachia group (S. aureum), the chief host is Lysimachia nummularia; 



