CHAPTER VIII. — THE THALLOPHYTA. 



3*3 



as in the rest, either antheridia are not produced at all, or these do 

 not perform the usual function of fertilization. The sexual pro- 

 cess has in fact degen- 

 erated into an asexual 

 one. This phenome- 

 non is particularly ob- 

 served in the genus 

 Saprolegnia, and, as we 

 shall presently see, is a 

 common one among 

 the higher groups of 

 the Fungi. 



Achlya lignicola, 

 which feeds upon 

 decaying, submerged 

 wood, illustrates well 

 the perfect forms of the 

 group. See Fig. 512 A 



and B. 

 a 



Fig. 512. — Achlya lignicola. A, Sporangia; a, sporangium nearly ripe but not yet dehis- 

 cent; b, sporangium dehiscing; z, zoospores. B, Stages in sexual reproduction. In i, the 

 oogonium, o, and the antheridia, a, a, not yet fully formed: 2, later stages when oogonium 

 is nearly ready for fertilization; 3, process completed and antheridia withering away. 

 Magnified about 300 diameters. 



CHAPTER VIII.— The Thallophvta. 



THE FUNGI (Continued). 



The Ascomycetes. 



This order consists, for the most part, of saprophytic hyphal 

 fungi, which produce a multicellular mycelium and on the whole 

 attain a complexity of structure exceeding that of any of the 

 groups already described. A few, however, as the Yeast-plant and 

 its allies, are among the simplest of vegetable organisms. Many 

 of tl\em reproduce by means of conidia, and these may be formed 

 from hyphse by the separation of cells in succession from their 

 free ends — a process called abjunction — or they may be produced 

 in special receptacles. But their most distinctive characteristic 

 is the production of ascosporcs. These are spores formed by 



