378 BOTANY 



Ascomycetes, cultivated without Algae and supplied with a proper 

 nutrient solution. 



In the formation of a fully-developed Lichen from the rudimentary 

 thallus (Fig. 302, 2) the hyphal tissue usually becomes differentiated 

 into a thick cortical layer of pseudo-parenchyma and into a more 

 loosely woven medullary layer, with the zone of gonidia entwined by 

 hyphae between the two. These different zones are most plainly 

 seen in the fruticose Lichens, among which the Beard Lichen, Usnea 

 barbata (Fig. 304), has developed in the medulla a mechanical system 

 consisting of a firm hyphal strand. Both the upper and under surfaces 

 of the foliaceous Lichens are usually covered with a cortical layer. 

 The medullary layer lies in the middle, between the two cortical 

 layers, while the gonidia form a layer between the upper cortex and 

 medulla. A cortical layer is present only on the upper side of most 

 foliaceous and crustaceous Lichens, or if present also on the under 

 side, it is developed merely on the margins ; the medullary layer then 

 lies directly upon the substratum. The thalli of the Lichens are 

 attached to the substratum by rhizoid hold-fasts, RHIZINES, which 

 consist of hyphae resembling root-hairs. 



Many Lichens are able to multiply in a purely vegetative manner, 

 by means of loosened pieces of the thallus, which continue their growth 

 and attach themselves to the substratum with new rhizines. The 

 majority of the heteromerous Lichens possess in the gonidial layer 

 another means of vegetative multiplication by forming soredia. 

 In this process, small groups of dividing gonidia become closely 

 entwined with mycelial hyphae, and form small isolated bodies which, 

 on the rupture of the thallus, are scattered in great numbers by the 

 wind and give rise to new Lichens. 



The fructifications of the Lichens are produced by the consorting 

 Fungi, not by the vegetating Algae. The Fungi belong chiefly to the 

 Discomycetes ; a few genera to the Pyrenomycetes ; and only a single 

 genus to the Hymenomycetes. In conformity with the nature of their 

 constituent Fungi, the first two groups are classified as Ascolichenes, 

 the third as Hymenolichenes. 



1. Ascolichenes 



(a) The Discolidienes or Lichenes gymiiocarpi produce, as the ascus-fruit of 

 their Fungus, chiefly cupular or discoid apothecia, sessile or somewhat depressed on 

 the thallus. In structure they resemble those of the Pezizeae (Fig. 282), and bear 

 on their upper side an hymenium of asci and paraphyses. One of the commonest 

 species of fruticose Lichens belonging to this group is Usnea barbata, the Beard 

 Lichen, frequently occurring on trees and having large, fringed apothecia (Fig. 

 304). Boccella tinetoria, another member of the DiscoMchenes, found widely 

 distributed on the rocks of the African coast and East Indies, has an erect, vermiform, 

 forked thallus from which litmus and orchil (orseille) are obtained. Oetraria 

 islandica, Iceland Moss (Fig. 305), occupies an intermediate position between the 

 fruticose and foliaceous Lichens. It has a divided, foliaceous but partially erect 



