ORDER LECANORALES (tHE DISKJLICHENS) 219 



acids is the result of mutual physiological interaction of alga and fungus, 

 as they do not occur in either organism Avhen groAvn alone. From some of 

 these acids may be obtained brilliant pigments such as orcein, litmus, 

 etc. (Miss Smith, 1926). 



M any lichens have no known asexual mode of reproducing themselves. 

 The pendent forms are frequently torn to pieces by the wind and carried 

 considerable distances, thus achieving distribution. A good many Hchens 

 produce pycnidia containing conidia. For some species these conidia have 

 been germinated to produce a mycelium. On a great many lichens special 

 asexual reproductive structures, the soredia, are produced. In barest 

 details a soredium arises as an outgrowth of the interior mycelium of the 

 thallus, carrying with it some of the algal cells. Having grown out through 

 the surface of the thallus this mycelial mass rounds up into a ball with 

 a sort of cortex, containing in its interior loose hyphae and a few host 

 cells. This structure breaks loose and is distributed by wind or rain to 

 other locations where the hyphae grow fast, thus starting a new lichen 

 thallus. 



In their sexual reproduction the Lecanorales are rather uniform in 

 their end product, the apothecium, which differs in detail but not in 

 fundamental plan in the various families and genera. These variations 

 ha\e to do with shape (concave, fiat, convex); color; structure of the 

 paraphyses; number, color, structure, and shape of the ascospores (color- 

 less or brown, one-celled or divided into two or more cells, ellipsoidal, 

 fusoid, needle-like, etc.); structure of the hypothecium and excipulum; 

 size; location on the thallus, etc. Two different structures called excipula 

 or exciples are of importance in systematic arrangement of the families 

 and genera of lichens. The "thalloid exciple" is a marginal wall around 

 the apothecium consisting of an upgrowth of the thalloid hyphae, often 

 with enclosed algal cells. It does not arise from the developing apothe- 

 cium. The "proper exciple" is the cup-like margin at the edge of the 

 apothecium which is formed by the outgrowth and upgrowth of the 

 apothecial tissues. Sometimes both types are present but more usually 

 only one type, and in many lichens neither type of exciple is observable. 

 In contrast with most of the Pezizales the apothecia of the Lecanorales 

 are usually slow in development and persist for a long time, maturing a 

 few asci at intervals. The asci are inoperculate and usually thickened at 

 the apex. The details of the sexual process, particularly the behavior of 

 the sexual nuclei, are sadly in need of further study in almost all genera 

 of the order. It may be safely said that no lichen has been satisfactorily 

 studied from all these standpoints. The conditions in Collema and Colle- 

 modes will illustrate the main features of the sexual process in this order. 



In its interior Collema consists of a slender, branched mycelium, 

 loosely penetrating and limited in outline to the shape of the Nostoc 



