n] 



ASCOMYCETES 



49 



brane which gradually closes in to produce, by further marginal growth, the 

 ellipsoidal plasma membrane of the spore. In this way the whole body of 

 the spore is cut out from the undifferentiated cytoplasm of the ascus by a 

 process of free cell formation, and its membrane is formed by the fusion of 

 the astral rays. 



A different account of, the development of the membrane was given by 

 Faull in 1905. According to his investigations the spore is cut out by the 

 gradual differentiation from the centrosome downwards of a limiting layer 

 of hyaline or finely granular cytoplasm, in the production of which the 

 astral radiations play no part. 



In many of the Discomycetes, however, the importance of the aster in 

 spore-formation is very marked, and the spore is outlined by radiations 

 passing out from the centrosome (fig. 17). These radiations, in Humaria 



9^■a< ■i^r:;i:l^i>^--^:':. 







Fig. 17. Humaria rutitans (Fr.) Sacc. ; stages of spore formation, 

 X 1875. 



rutilans, Peziza vesiculosa, or Lachnea stercorea, doubtless indicate the paths 

 of altered substances emanating from the centrosome as a centre of activity, 

 and flowing back past the nucleus as the developing beak pushes into the 

 cytoplasm. As these substances increase a membrane is formed, and the 

 spore, or the part of it near the centrosome, is cut out. In the delimitation 

 of the region remote from the centrosome the vacuoles in the cytoplasm of 

 the ascus may take part. These vacuoles are especially plentiful in Ascobolus 

 furfuraceus, and in this fungus their share in spore-formation is important. 

 After the spore is delimited the differentiation of its wall is largely due to 

 the epiplasm, and is not complete till the ascus is almost ripe. 



Phylogeny. The recognition of the specialized character of the ascus 

 has led to a general assumption of the monophyletic origin of the Ascomy- 

 cetes, and speculation as to their possible ancestry has run along two main 

 lines. They have been regarded as derived eitherfroma phycomycetous group, 

 or from the Red Algae or the ancestors of the latter. An independent origin 

 either among the flagellates or the Green Algae has also been proposed. 



The suggestion of a floridean relationship was first made by Sachs, in 

 1874, and has recently been supported by Harper, Dodge ^ and others; the 



' Bull. Torrey Bot. Club, 1914, xli, p. 157. 

 G.-V. A 



