560 COMPARATIVE MORPHOLOGY OF FUNGI 



swell greatly, form a loose pseudoparenchyma poor in cytoplasm, and 

 begin to degenerate slowly (Fig. 376, 1). This swelling and degeneration 

 of the cells proceeds basipetally, until the pseudoparenchyma includes 

 two-thirds to three-fourths of the knot and only 4 to 5 of the basal 

 cells of the hyphae remain. These are filled with cytoplasm and are 

 apparently richly nourished at the expense of the disintegrating terminal 

 cells. Between each cell of a hypha and a cell of a neighboring hypha, 

 follows a fusion of protoplasts, as in the caeoma, by the solution of the 

 separating wall at first in the center, later in the periphery of the sorus. 

 Usually only one cell of a hypha takes part in plasmogamy; but all are 

 potentially capable, since the fusion cells lie scattered irregularly (i.e., 

 at unequal heights) over the basal tissue and it is impossible to predict 

 which cells are active. In addition, two cells lying one above the other 

 may copulate with the same hypha. The further development of the 

 fusion cells, the cutting off of the initial cells and their differentiation 

 into aeciospores and intercalary cells, takes place in the same manner 

 as in the caeoma. The growing spore chains push the pseudoparen- 

 chyma in front of them and press against the epidermal layer of the 

 host tissues. 



The type of Uredinopsis americana, to which belong Thekopsora 

 Vacciniorum (Pucciniastrum Myrtilli) (Adams, 1919) and Gijmnosporan- 

 gium juniperinum (Kursanov, 1922), resembles Puccinia Pruni-spinosae 

 in the differentiation of the hyphal knot. As in the latter, the 

 intercalary cells swell up; over the degenerated, gelatinous pseudo- 

 parenchyma there lies another layer of compact vegetative tissue which 

 separates the pseudoparenchyma from the host tissue. 



The third type, which includes Puccinia Violae (Fromme, 1914; Mme. 

 Moreau, 1914; Kursanov, 1922), P. Falcariae (Dittschlag, 1910) and P. 

 graminis (Kursanov, 1922) resembles P. Mariae-Wilsoni except that in 

 this type the degeneration of the hyphae proceeds so far that there 

 remains only a single layer of potential sexual cells. This cell layer 

 corresponds in appearance to the palisade layer of the caeoma, only it 

 lies inside the hyphal knot instead of on its top. Just as in the palisade 

 cells of the caeoma, these sexual cells occasionally cut off some sterile 

 cells, a thing not yet observed in P. Mariae Wilsoni. 



In the fourth type, to which, for example, belong Endophyllum Semper- 

 vivi (Hoffmann, 1912) and Puccinia Eatoniae (Fromme, 1914) develop- 

 ment is still simpler. Here, on the whole, no special sexual cells are 

 formed, but plasmogamy takes place between any two mycelial cells in 

 the basal region of the plectenchymatic knot, where the fusion cells 

 elongate radially and cut off chains of initial cells as basal cells. 



In certain cases, it has been demonstrated that at the base of the 

 aecia plasmogamy rarely takes place; in certain strains of Endophyllum 

 Euphorbiae-sijlvaticae (Mme. Moreau, 1914), of E. Centranthi-rubri 



