146 



MORPHOLOGY 



narrower than the others and are very frequently raised from contact with 

 the substratum. They tend to grow out from the thallus in an upright 



Fig. 84. Parmelia physodes Ach. Thallus growing ver- 

 tically ; soredia chiefly on the lobes directed downwards, 

 reduced (M. P., Photo.}. 



direction and then to turn backwards at the tip, so that the opening of the 

 soralium is directed downwards. Bitter says that the cause of this change 

 in direction is not clear, though possibly on teleological reasoning it is of 

 advantage that the opening of the soralium should be protected from direct 

 rainfall. The opening lies midway between the upper and lower cortex, and 

 the upper tissue in these capitate soralia continues to grow and to form an 

 arched helmet or hood-covering which serves further to protect the soralium. 



Similar soralia -are characteristic of Physcia Jiispida {Ph. stellaris subsp. 

 tenella), the apical helmet being a specially pronounced feature of that species, 

 though, as Lesdain 1 has pointed out, the hooded structures are primarily 

 the work of insects. In vertical substrata they occur on the lower lobes of 

 the plant. 



Apical soralia are rare in fruticose lichens, but in an Alpine variety of 



1 Lesdain 1910. 



