LICHENES. 



295 



study of tlie youngf and lorming knot, while the sucoeedinij ones will 

 show first the conidia, and then the forming perithecia and developing 

 asci and ascospores. The last gathered specimens in February will 

 show the fully formed ascospores. 



(e) Ergot, which occurs on rye and many of the forage grasses, is 

 poisonous, producing gangrenous sores when eaten in considerable 

 quantities. It is used somewhat in medicine. 



(/) Xylomitea in the Jurassic, and BpJiceria, Phacidmm, Rhytisma 

 and other genera, in the „ ^ 



Eocene and Miocene, are 

 the fossil representatives 

 of this order. 



391.— Order Licli- 

 enes. Lichens agree, 

 in all the essentials of 

 their structure, with 

 thetwoprecediag or- 

 ders, HelvellacccB and 

 Pyrenomycetes, and 

 there can no longer 

 be shown any good 

 reasons for not class- 

 ing them with the 

 latter, under the As- 

 comycetes. 



392. — The tissues 

 of lichens consist of 

 various aggregations 

 of colorless, jointed 

 hyphse ; in general 

 the hyphsB in the cor- 

 tical portion of the 

 thallus are compact- 

 ed and developed into a pseudo-parenchyma (o and u, Fig. 301, 

 and cc, B, Pig. 303), while in the medullary portion they are 

 distinct {m. Pig. 301, and cm, B, Pig. 303). In all lichens 

 there occur numerous green, blue-green, or brown-green cells, 

 the gonidia, which are either scattered through the interior 

 (homoomerous) , or disposed in one or more distinct layers 

 (heteromerous) ; of the former, Collema and Leptogium are 



Fig. 201.— Transverse section of the thallns of 

 Sticta fuliginosa, o, cortical layer of rlie upper sur- 

 face ; M. cortical layer tjf lower surface ; r, rhizoidd 

 or attaching fibres ; m, medullary layer, composed of 

 distinct hyphie, many of which are cut transversely; 

 .9, layer of green gonidia. Bach gonidia group is sur- 

 rounded by a gelatinous envelope. X 650.— After 

 Sachs. 



