STRATOSE THALLUS 87 



which the cortex is specially adapted are the checking of transpiration and 

 the strengthening of the thallus against external strains. 



d. Cortical Hairs or Trichoma. Though somewhat rare, cortical 

 hairs are present on the upper surface of several foliose lichens. They take 

 rise, in all the instances noted, as a prolongation of one of the cell-rows 

 forming a plectenchymatous cortex. 



In Peltidea {Peltigerd) aphthosa they are especially evident near the 

 growing edges of the thallus; and they take part in the development of 

 the superficial cephalodia^ which are a constant feature of the lichen. They 

 tend to disappear with age and leave the central older parts of the thallus 

 smooth and shining. In several other species of Peltigera {P. canina, etc.) 

 they are present and persist during the life of the cortex. In these lichens 

 the cells of the cortical tissue are thin-walled, all except the outer layer, 

 the membranes of which are much thicker. The hairs rising from them are 

 also thick-walled and septate. Generally they branch in all directions and 

 anastomose with neighbouring hairs so that a confused felted tangle is 

 formed; they vary in size but are, as a rule, about double the width of the 

 medullary hyphae as are the cortical cells from which they rise. They disap- 

 pear from the thallus, frequently in patches, probably by weathering, but 

 over large surfaces, and especially where any inequality affords a shelte^ 

 they persist as a soft down. 



Hairs are also present on the upper surface of some Parmeliae. Rosen- 

 dahl^ has described and figured them in P. glabra and P. verriiculifera — 

 short pointed unbranched hyphae, two or more septate and with thickened 

 walls. They are most easily seen near the edge of the thallus, though they 

 persist more or less over the surface; they also grow on the margins of the 

 apothecia. In P. verruculifera they arise from the soredia; in P. glabra 

 a few isolated hairs are present on the under surface. 



In N ephromunn tomentosum there is a scanty formation of hairs on the 

 upper surface. They are abundant on the lower surface, and function as 

 attaching organs. A thick tomentum of hairs is similarly present on the 

 lower surface of many of the Stictaceae either as an almost unbroken 

 covering or in scattered patches. In several species of Leptogium they grow 

 out from the lower cortical cells and attach the thin horizontal fronds; and 

 very occasionally they are present in Collema. 



C. GoNiDiAL Tissues 



With the exception of some species of Collema and Leptogium lichens 

 included under the term foliose, are heteromerous in structure, and the algae 

 that form the gonidial zone are situated below the upper cortex and, there- 



' See p. 133. 2 Rosendahl 1907. 



