STRUCTURES PECULIAR TO LICHENS 145 



with the exception of the Pertusariaceae, where they are frequent, and some 

 species of Lecanora and Placodium. They are known in only two hypo- 



Fig. 83. Parmelia physodes Ach. Thallus growing horizontally ; soredia on 

 the ends of the lobes (S. H. , Photo.}. 



ph\oeoda\(subcortica\')\\chens,Ar^onta^rumosa and Xylographa spilomatica. 

 Among squamulose thalli they are typical of some Cladontae, and also of 

 "Lecidea (Psora) ostreata, where they are produced on the upper surface to- 

 wards the apex of the squamule. 



b. POSITION OF SORALIFEROUS LOBES. According to observations'made 

 by Bitter 1 , the occurrence of soralia on one lobe or another may depend to 

 a considerable extent on the orientation of the thallus. He cites the varia- 

 bility in habit of the familiar lichen, Parmelia physodes and its various forms, 

 which grow on trees or on soil. In the horizontal thalli there is much less 

 tendency to soredial formation, and the soredia that arise are generally 

 confined to branching lobes on the older parts of the thallus. 



That type of growth is in marked contrast with the thallus obliged to 

 take a vertical direction as on a tree. In such a case the lobes, growing 

 downward from the point of origin, form soralia at their tips at an early 

 stage (Fig. 84). The lateral lobes, and especially those that lie close to the 

 substratum, are the next to become soraliate. Similar observations have 

 been made on the soraliferous lobes of Cetraria pinastri. The cause is 

 probably due to the greater excess of moisture draining downwards to the 

 lower parts of the thallus. The lobes that bear the soralia are generally 



1 Bitter 1901. 



S. L. 



10 



