144 MORPHOLOGY 



genus Usnea which give rise to new branches. Many of the species in that 

 genus are plentifully sprinkled with the white powdery bodies. A short 

 way back from the apex of the filament the separate soredia show a tendency 

 to apical growth and might be regarded as groups of young plants still 

 attached to the parent branch. One of these developing more quickly 

 pushes the others aside and by continued growth fills up the soredial 

 opening in the cortex with a plug of tissue; finally it forms a complete 

 lateral branch. Schwendener calls them "soredial" branches (Fig. 82) to 

 distinguish them from the others formed in the course of the normal 

 development. 



B. SORALIA 



In lichens of foliose and frutrcose structure, and in a few crustaceous 

 forms, the soredia are massed together into the compact bodies called soralia, 

 and thus are confined to certain areas of the plant surface. The simpler 

 soralia arise from the gonidial zone below the cortex by the active division 

 of some of the algal cells. The hyphae, interlaced with the green cells, are 

 thin-walled and are, as stated by Wainio^, still in a meristematic condition ; 

 they are thus able readily to branch and to form new filaments which clasp 

 the continually multiplying gonidia. This growth is in an upward or out- 

 ward direction away from the medulla, and strong mechanical pressure is 

 exerted by the increasing tissue on the overlying cortical layers. Finally 

 the soredia force their way through to the surface at definite points. The 

 cortex is thrown back and forms a margin round the soralium, though shreds 

 of epidermal tissue remain for a time mixed with the powdery granules. 



a. Form and Occurrence of Soralia. The term "soralium" was 

 first applied only to the highly developed soredial structures considered by 

 Acharius to be secondary apothecia; it is now employed for any circum- 

 scribed group of soredia. The soralia vary in size and form and in position, 

 according to the species on which they occur; these characters are constant 

 enough to be of considerable diagnostic value. Within the single genus 

 Pftrmelia, they are to be found as small round dots sprinkled over the 

 surface of P. dubia; as elongate furrows irregularly placed on P. sulcata; as 

 pearly excrescences at or near the margins of P. perlata, and as swollen 

 tubercles at the tips of the lobes of P.physodes (Fig. 83). Their development 

 is strongly influenced and furthered by shade and moisture, and, given such 

 conditions in excess, they may coalesce and cover large patches of the thallus 

 with a powdery coating, though only in those species that would have borne 

 soredia in fairly normal conditions. 



Soralia of definite form are of rather rare occurrence in crustaceous lichens, 



1 Wainio 1897, p. 32. 2 Reinke 1895, p. 380. 



