106 MORPHOLOGY 



fastigiate cortex as in Roccella. Bending strains to which the same lichens 

 are subjected, are equally well met by the peripheral disposition of the 

 mechanical elements. 



Tenacity and elasticity are provided for in the pendulous forms either 

 by a fibrous cortex as in Alectoria, or by the chondroid axis in Usnea. 

 Haberlandt 1 has recorded some interesting results of tests made by him as 

 to the stretching capacity of a freshly gathered pendulous species in which 

 the central strand was from *5 to I mm. thick. He found he could draw it 

 out 100 to 1 10 per cent, of its normal length before it gave way. In an 

 upright species the frond broke when stretched 60 to 70 per cent. In both 

 of the plants tested, the central strand retained its elasticity up to 20 per 

 cent, of stretching. The outer cortical tissue was cracked and broken in 

 the experiments. Schulte 3 calculated somewhat roughly the tenacity of 

 Usnea longissima and found that a piece of the main axis 8 cm. long carried 

 up to 300 grms. without breaking. 



D. RETICULATE FRONDS 



In the upright radiate thallus, more especially among the Ramalinae, 

 though also among Cladoniae*, there has appeared a reticulate thallus resulting 

 from the elongate splitting of the tissues, and due to unequal growth tension 

 and straining of the gelatinous cortex when swollen with moisture. In 

 several species of Ramalina, the strap-shaped frond is hollow in the centre ; 

 and strands of strengthening fibres give rise to a series of cortical ridges. 

 The thinner tissue between is frequently torn apart and ellipsoid openings 

 appear which do not however pierce beyond the central hollow. Such breaks 

 are irregular and accidental though occurring constantly in Ramalina 

 fraxinea, R. dilacerata, etc. 



A more complete type of reticulation is always present in a Californian 

 lichen, Ramalina reticulata, in which the large flat frond is a delicate open 

 network from tip to base (Fig. 64). It grows on the branches of deciduous 

 trees and hangs in crowded tufts up to 30 cm. or more in length. Usually 

 it is so torn, that the real size attainable can only be guessed at. It is 

 attached at the base by a spreading discoid hold-fast, and, in mature plants, 

 consists of a stoutish main axis from which side branches are irregularly 

 given off. These latter are firm at the base like the parent stalk, but soon 

 they broaden out into very wide fronds. Splitting begins at the tips of the 

 branches while still young ; they are then spathulate in form with a slightly 

 narrower recurved tip, below which the first perforations are visible, small at 

 first, but gradually enlarging with the growth of the frond. 



Ramalina reticulata is an extremely gelatinous lichen and the formation 



1 Haberlandt 1896. z Schulte 1904. s See p. 120. 



