64 THE THALLUS OF THE GENUS PARHELIA, 



tudes of liyphse lying parallel to the long axis of the 

 rootlet, derived from the hyphae of the medulla, and en- 

 cased by a continuation of the cortical threads, wbicli 

 coat them externally, and when they are hollow as in 

 F. liinhata and P. cetrata, also form an inner layer to 

 the tubular rootlet. Xlie walls of the rhizinal hyphse are 

 densei, and the th]"eadsi are by no means discrete asi in the 

 genus Sticta. The outer threads have the thick walls and 

 necklace-like structure of thei cortical hyphce, the lumen 

 of each cell being small in contrast to its diameter. 



In the central threads which show no division into 

 cellules, the so-called "micro-gonidia" can be clearly dis- 

 tinguished. 



FarmeUa ijerforata differs from its allies in having 

 rhizina3 wholly ooimposed of hyph^ made up of tubular 

 cellules whose course in the rootlet is less parallel than 

 those of kiiidred species. 



The Parmeliasi inhabit stones, rocks, fences, and trees, 

 in situations where there are, in Australia, strong con- 

 trasts of temperature and rainfall. The Stictas inhabit 

 scrubs and brushes, where shade and rain are plentiful. 

 With the steady drip of rain and dew on the tree trunks 

 and branches, moisture is always available to' a. Sticta, and 

 its rhizinal system is simple and delicate. The Parmelia 

 has to face more difficult circumstances; its rhizinas are 

 thicker, the threads have stronger walls to prevent desicca- 

 tion, the hyphie are more numerous to^ take advanta^je of 

 every particle of water, and to cling to every crevice, and 

 the cortical system of each rhizina keeps the unseptate 

 threads from loss of imbibed water by evaporation, and 

 from interference with their functions. 



It has been shown that, in two species at least, P. 

 limhata and P. cetrata, the hollow rhizinae are in connection 

 with absorption poxes, and this gives an additional reason 

 for believing the pores to be used for taking in liquid food. 



6. The Spermagonia. 



These are small oonceptacles, sunk in the thallus, and 

 varying from circular to ovate or obovate in transverse sec- 

 tion, ranging from .3-. 7 mm. in diameter. They arise as 

 •small coils of hyphse in the medullary part of the thallus, 

 without pores or connection with the upper or lower cor- 

 tex. At first occupying a central position, growth develops 

 more rapidly on the side towards the upper cortex. The 

 liyphae forming the wall of the spermagonium are brown and 



