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I. GROWTH OF THE THALLUS. 



(a) The Protothallus {Hyfothallus, Promycelium). 



As already indicated, the organ of lichens designated by the 

 names protothallus, hypothallus or promycelium is, perhaps, morpho- 

 logically unrecognizable ; it will receive some consideration because 

 in various works on lichenology it has been considered of great im- 

 portance, especially in classification. 



According to Schwendener the protothallus is the product of the 

 spore and is said to occur only in crustaceous or scaly lichens, ap- 

 pearing as a hyphal tissue forming a fringe about the thallus proper. 

 It usually differs from the thallus in color ; it is most frequently black ; 

 the hyphae extend radially, that is, in the direction of growth of the 

 thallus. 



These observations of Schwendener are in the main correct, 

 though their true significance is apparently not rightly understood. 

 The protothallus is neither more nor less than the spore-product of 

 the hyphal symbiont extending over the substratum. The hyphae 

 form the apical growth of the thallus ; the lower filaments extend into 

 the substratum forming the rhizoidal network, the upper surround 

 the algae at some distance from the margin. The color, which is 

 usually well marked in old specimens, depends largely upon the 

 substratum. In some cases there is a marked difference between the 

 hyphae of the " protothallus" and the hyphae of the thallus, which 

 seems to favor the belief that the protothallus is something foreign. 

 It is quite probable that this doubtful structure is the hyphal network 

 of a fungus preceding the growing lichen-thallus with which it 

 has formed a symbiotic association ; Minks believes that in many 

 cases the protothallus is formed by the reduced host-lichen upon 

 which some other lichen exists as a parasite. 



No protothallus can be detected in mature higher lichens. The 

 first product of spore development before it enters into symbiotic as- 

 sociation with the algae has been designated as protothallus, especially 

 by Schwendener. The term promycelium would seem more appro- 

 priate because of the fact that no thallus is yet formed. It can readily 

 be understood that the term promycelium must likewise be variable 

 and uncertain in its application. Sometimes the first hyphae come in 

 contact with the algae immediately, sometimes not at all, in which 

 case one of two things may happen ; if it is a low form of lichen 

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