48 ASCOMYCETES [CH. 



cause of the second fusion in the ancestors of our present Ascomycetes, it 

 is clear that in the forms now extant the existence of a second reduction 

 must be an important factor; in this respect the organism may well be 

 moving in a vicious circle. 



Pseudapogamy. The above, at any rate, seems to be the case in respect of 

 the much more important fusion of the sexual nuclei. In Humaria granulata 

 no antheridium is developed, and the female nuclei, as recorded by Blackman 

 and Fraser in 1906, fuse in pairs in the oogonium before passing into the 

 ascogenous hyphae. The same state of affairs was observed by Welsford in 

 Ascobolus furfuraceus in 1907, and by Cutting in Ascophanus carneus and 

 Dale in Eurotium repens in 1909; it is difficult to see how any important 

 physiological benefit can ensue from the union of closely related nuclei 

 developed in the same cell; the determining cause would seem to be the 

 need of preparation for the meiotic phase already established in the life- 

 history. In Lachnea stercorea an antheridium is present and fuses with the 

 terminal cell of the multicellular trichogyne, but the male nuclei never 

 reach the oogonium, and here also the female nuclei unite in pairs. 



In Humaria rutilans matters have gone still further; not only is the 

 antheridium lacking but the archicarp also is not developed. Nuclear fusion, 

 sometimes preceded by the migration of one of the fusing nuclei, takes place 

 in the vegetative cells of the developing apothecium. The cells containing 

 fusion nuclei give rise to ascogenous hyphae, while those in which fusion has 

 not occurred produce the paraphyses and the sheath. A similar state of affairs 

 has been reported by Carruthers in Helvetia crispa, and evidence of its occur- 

 rence in Polystigma rubrum has been noted by Blackman and Welsford. 



These reduced forms, belonging respectively to de Bary's categories of 

 parthenogenesis and apogamy, have thus proved to be pseudapogamous 

 in the sense of Farmer and Digby 1 , since in them normal fertilization is re- 

 placed by the union in pairs of female or vegetative nuclei. Meiosis takes 

 place as usual. 



Spore-Formation. After the third division in the ascus, preparations 

 for spore-formation begin. This stage was first described in detail by Harper 

 in 1895, and was subsequently elucidated by him in other papers, and 

 especially in his very full study of the mildews in 1905. As the third mitosis 

 comes to an end the eight daughter nuclei, or those of them about which 

 spore-formation is to take place, become pear-shaped, a beak being pushed 

 or pulled out from each; the centrosome lies at the tip of the beak, and from 

 it spread the astral rays, to the activity of which Harper is inclined to attri- 

 bute the formation of the beak. As development proceeds, these rays become 

 folded over so that they extend past the nucleus, and Harper describes them 

 as combining side by side to form a continuous, broad, umbrella-like mem- 



1 Ann. Bot. 1907, xxi, p. 191. 



