io8 MORPHOLOGY 



of the network was supposed by Lutz 1 to be entirely due to the swelling of 

 the tissues, or the imbibition of water, causing tension and splitting. A more 

 exact explanation of the phenomenon is given by Peirce 2 : he found that it 

 was due to the thickened incurved tip, which, on the addition of moisture, 

 swells in length, breadth and thickness, causing it to bend slightly upwards 

 and then curve backwards over the thallus, thus straining the part imme- 

 diately behind. These various movements result in the splitting of the frond 

 while it is young and the cortices are thin and weak. 



Peirce made a series of experiments to test the capacity of the tissues 

 to support tensile strains. In a dry state, a piece of the lichen held a weight 

 up to 1 50 grms. ; when wet it broke with a weight of 30 grms. It was also 

 observed that the thickness of the frond doubled on wetting. 



E. ROOTING BASE IN FRUTICOSE LICHENS 



Fruticose and filamentous lichens are distinguished by their mode of 

 attachment to the substratum : instead of a system of rhizinae or of hairs 

 spread over a large area, there is usually one definite rooting base by which 

 the plant maintains its hold on the support. 



Intermediate between the foliose and fruticose types of thallus are 

 several species which are decumbent in habit, but which are attached at one 

 (or sometimes more) definite points, with but little penetration of the under- 

 lying substance. One such lichen, Evernia furfuracea, has been classified 

 now as foliose, and again as fruticose. The earliest stage of the thallus is 

 in the form of a rosette-like sheath which bears rhizinae on the under 

 surface, very numerous at the centre of the sheath, but entirely wanting 

 towards the periphery. A secondary thallus of strap-shaped rather narrow 

 fronds rises from the sheath and increases by irregular dichotomous branch- 

 ing. These branches, which are considered by Zopf 3 as adventitious, may 

 also come into contact with the substratum and produce a few rhizinae at 

 that point; or if the frond is more closely applied, the irritation thus 

 produced causes a still greater outgrowth of rhizinae and the formation of 

 a new base from which other fronds originate. These renewed centres of 

 growth are not of very frequent occurrence; they were first observed and 

 described by Lindau 4 in another species, Evernia prunastri, and were aptly 

 compared by him to the creeping stolons of flowering plants. 



Evernia furfuracea.grows frequently on dead wood, palings, etc., as well 

 as on trees. E.prunastri grows invariably on trees, and has a more constantly 

 upright fruticose habit; in this species also, a basal sheath is present, and 

 the attachment is secured by means of rhizoidal hyphae which penetrate 

 deeply into the periderm of the tree, taking advantage of the openings 



1 Lutz 1894. 2 Peirce 1898. 3 Zopf 1903. 4 Lindau 1895. 



