92 LICHENS. 



the thallus being flat, lobed, and scalloped (crenate) 

 at the margins. The structure of the thallus may 

 serve to represent that of most of the Lichens. It 

 consists of three layers, an upper cortical or epider- 

 mic layer, which is continued over the margins to the 

 under surface of the thallus, to form the under layer ; 

 and between these is the middle or medullary layer. 

 The medullary layer consists of interwoven fibres, 

 which are more closely packed towards the upper and 

 under surfaces, so as to give them a cellular appear- 

 ance. Near the upper part of the medullary layer, 

 a number of minute rounded green cells exist, lying 

 loosely in its meshes (PI. II. fig. 4 a). These green 

 cells (gonidia) appear to correspond to the buds of 

 the higher plants, and, when detached from the plants, 

 they are capable of growing into new individuals. 



On the upper surface of the thallus of Parmelia 

 (fig. 2) the fructification may be observed. This 

 consists of saucers or shields (apothecia, airoBriK^ a 

 repository), formed of a raised and expanded portion 

 of the thallus (fig. 3), and containing the spores. 

 The spores are enclosed in closely set upright cells, 

 or spore-sacs (figs. 4 b and 5 b), called as'd (acro9, a 

 bottle) or thecse ; and intermingled with the asci are 

 filaments, enlarged and coloured at the ends (para- 

 physes), which are probably abortive asci. 



In systematic works upon the Lichens, the saucers 

 and their contents are included in the term apothecia, 

 the saucer alone being called theexcip'ulum (excipulum, 

 a receiver) ; the mass of asci and paraphyses forming 

 the nucleus or thaldmium (OdXafios, a bed) . 



The yellow spores are very minute, each ascus con- 

 taining eight of them, and they are divided by a trans- 

 verse partition or septum. 



Near the margins of the lobes of the thallus are 

 small dark points. These are the pouting mouths of 

 little capsules (spermogonia) sunk in the substance 

 of the thallus, and containing numerous filaments, 



