i io MORPHOLOGY 



a crustaceous thallus. It is possible also to see on the rock, here and there, 

 small areas of compact thalline granules that have scarcely begun to put out 

 the upright fronds. These granules are corticate on the upper surface and 

 contain gonidia; from the lower surface, slender branching hyphae in rhizoid- 

 like strands penetrate down between the inequalities and separable particles 

 of the rock, if the formation is granitic. They frequently have groups of 

 gonidia associated with them, and they continue to ramify and spread, the 

 pure white filaments often enough enclosing morsels of the rock. The 

 upright fronds are continuous with the base and are thus securely anchored 

 to the substratum. 



On a smooth rock surface such as quartzite a continuous sward of Rama- 

 Una growth is impossible. The basal hyphae being unable to penetrate the 

 even surface of the rock, the attachment is slight and the plants are easily 

 dislodged. They do however succeed, sometimes, in taking hold, and small 

 groups of fronds arise from a crustaceous base which varies in depth from 

 5 to I mm. The tissues of this base are very irregularly arranged : towards 

 the upper surface loose hyphae with scattered groups of algae are traversed 

 by strands. of gelatinized sclerotic hyphae similar to the strengthening tissues 

 of the upright fronds, while down below there are to be found not only 

 slender hyphae, but a layer of gonidia visible as a white and green film on 

 the rock when the overlying particles are scaled off. 



Darbishire 1 found that attachment to the substratum by means of a 

 basal sheath was characteristic of all the genera of Roccellaceae. He looks 

 on this sheath, which is the first stage in the development of the plant, as 

 a primary or proto-thallus, analogous to the primary squamules of the 

 Cladoniae, and he carries the analogy still further by treating the upright 

 fronds as podetia. The sheath of the Roccellaceae varies in size but it is 

 always of very limited extent; it is mainly composed of medullary hyphae, 

 and gonidia may or may not be present. The whole structure is permanent 

 and important, and is generally protected by a well-developed upper cortex 

 similar in structure to that of the upright thallus, i.e. of a fastigiate type. 

 There is no lower cortex. 



The two British species of Roccella R. fuciformis and R. phycopsis 

 grow on maritime rocks, the latter also occasionally on trees. In R. fuci- 

 formis, the attaching sheath is a flat structure which slopes up a little round 

 the base of the upright frond. It is about 2 mm. thick, the cortex occupying 

 about 40 fi of that space; a few scattered gonidia are present immediately 

 below. The remaining tissue of the sheath is composed of firmly wefted 

 slender filaments. Towards the lower surface, there is a more closely com- 

 pacted dark brown layer from which pass out the hyphae that penetrate 

 the rock. 



1 Darbishire 1898. 



