SECT. 1 CRYPTOGAMS 407 



species from the one on which the teleutospores were prodnced), giving rise to an 

 interceUular mycelium, all the cells of which are uninucleate. From this 

 mycelium organs of two kinds arise, spermogonia on the upper surface of the leaf 

 and aecidia on the lower surface. 



The SPERMOGONIA (Fig. 339) are flask-shaped structures, the base of which is 

 covered with the projecting ends of hypliae ; from these are abstricted spermatia, 

 each of which has a single nucleus. Morphologically they are completely com- 

 parable to the similarly named male sexual organs of some Ascomycetes ; among 

 the Basidiomycetes they persist only in the Uredineae, and even in them are no longer 

 functional and may be completely wanting. In nutrient solutions the spermatia 

 may put out short germ-tubes, but are not capable of infecting the host plant. 



The AECIDIA (Fig. 340) are cup-shaped fructifications, which when young are 

 closed, but later open ; from the ends of the hyjihae numerous closely associated 

 chains of spores are abstricted. As a rule the enveloping layer or peridium of the 

 aecidium is formed of thick-walled cells corresponding to the sterilised peri})heral 

 rows of spores. In Phragmidium violaceum, which occurs on Blackberry leaves 

 and has been fully investigated by Blackmak (®^), the hyphae beneath the epidermis 

 when about to give rise to an aecidium fii-st cut off a sterile cell, which undergoes 

 no further development, from their ends (Fig. 341, A). The cell below this increases 

 in size ; it has at first only a single nucleus, but becomes binucleate by the passage 

 of a nucleus into it from an adjoining mycelial cell. The two nuclei do not fuse. 

 The binucleate cell undergoes successive divisions into a chain of spore-mother-cells, 

 each of which has a jjair of nuclei ; and from each spore-mother-cell an upper 

 binucleate aecidiospore and a sterile intercalary cell, which is also binucleate but 

 soon shrivels up, are derived by a transverse division {B, C). 



According to Christman (*'') the development of the aecidiospores in Phrag- 

 midium specios'um (Fig. 342), which is parasitic on Rosa, proceeds somewhat 

 differently. Here also the ends of the hyphae {A) divide into a terminal sterile cell 

 and a lower fertile cell {B), but the fertile cells fuse in pairs with one another, 

 the upper portions of the separating walls breaking down (6'). The two nuclei lie 

 side by side and divide simultaneously (conjugate division). Two of the daughter- 

 nuclei remain in the lower part and two pass to the upper portion of the dividing 

 cell, and this upper portion is separated by a transverse wall as the first spore- 

 mother-cell [D). In other respects the formation of the aecidiospores proceeds as 

 described above. A peridium is not formed in Plvragmidmyn. 



The ripe, binucleate aecidiospores (Fig. 341, D) are shed and infect a new host 

 plant. Each spore gives rise to an intercellular mycelium which soon proceeds in 

 the summer to bear uredospores or summer spores. These appear in small circular 

 or linear groups and arise singly from the enlarging terminal cells of the hyphae 

 (Fig. 338, 5, 6). They have two nuclei like all the cells of the mycelium developed 

 from the aecidiospore. They serve commonly to ensure the spread of the fungus in 

 the summer. Later, either in the same or in distinct sori the teleutospores are 

 formed, and in these the fusion of the two nuclei to a single one takes place ; such 

 a fusion as a rule is found to take place in the young basidium. 



The two types of cell fusion in the formation of the aecidium are also known in 

 other Uredineae, and must be regarded as replacing a formerly existing method of 

 fertilisation. If we attempt to derive the Uredineae from the Ascomycetes the 

 spermatia must be regarded as now functionless male cells, and the so-called fertile 

 cells in the young aecidium as corresponding to carpogonia. Extending the com- 

 parison further the mycelium proceeding from tiie aecidiospore in the Uredineae 

 and the uredospores and teleutospores borne on it, together with the basidia, 



2d 2 



