DIV. I 



THALLOPHYTA 



447 



the end of the ascogenous hypha when about to form an ascus becomes curved 

 into a hook-like shape (Fig. 391 A}. The two nuclei of the young ascus (a) lie 

 near to the bend, and on the formation of transverse walls are separated from 

 the uninucleate terminal cell (/&) and the stalk-cell (s), which also has a single 

 nucleus. The two nuclei of the young ascus fuse (C], and the resulting nucleus 

 gives rise by repeated division to the nuclei of the eight ascospores (D). The 

 terminal cell of the hook (h) and the stalk-cell (s) have their cavities continuous, 

 so that a binucleate fusion cell results which can proceed to form another young 

 ascus. In this way complicated systems of ascogenous hyphae arise. 



The highest development is exhibited by the peculiar fructifications of the 

 Helvellaceae, the mycelium of which grows in the humus soil of woods. In the 



FIG. 391. Development of the Ascus. A-C, 

 Pyronema confluens. (After HARPER.) 

 D, Young ascus of Boudiera with eigkt 

 spores. ( After CLAUSSEX.) Explanation 

 in text. 



FIG. 392. Morchella esculenta. 

 (f nat. size.) 



genus Morchella (Fig. 392) the fructifications consist of a thick erect stalk, bearing a 

 club-shaped or more or less spherical cap or pileus, which bears the hymenium, 

 with the eight-spored asci, on the reticulately-indented exterior surface (Fig. 381). 

 The Morchellas are edible ( 70 ), in particular 31. esculenta and M. conica. The former 

 has a yellowish -brown cap, ovately spherical in shape, and attains a height of 

 12 cm. ; the cap of the latter is conical and dark brown, and it reaches a height 

 of 20 cm. Gyromilra esculenta, with dark brown cap and white stalk, and others 

 are also edible. In their external appearance the fructifications of these highly 

 developed Discomycetes greatly resemble those of the Basidiomycetes. 



Order 5. Tuberaceae (Truffles) ( n ) 



The Tuberaceae or Truffle Fungi are saprophytic Ascomycetes, the mycelium 

 of which occurs in humus soil, particularly in woods. The ascus fructifications 

 familiar under the name of truffles are underground tuberous bodies, consisting 

 of a thick, investing layer, with passages opening to the exterior ; the walls of these 

 are lined with the hymenium composed of club-shaped asci (Fig. 393). The asci 

 contain only a small number of spores; in the case of the true Truffles (Tuber) 

 they are usually only four in number, and generally have a spinous or reticulately- 



