STRUCTURES PECULIAR TO LICHENS 147 



Ramalina minuscula they are formed at the tips of the fronds and are pro- 

 tected by an extension of the upper cortical tissues. Another instance occurs 

 in a Ramalina from New Granada referred by Nylander to R. calicaris var. 

 farinacea; it presents a striking example of the helmet tip. 



c. DEEP-SEATED SORALIA. In the cases already described Schwendener 1 

 and Nilson 2 held that the algae gave the first impulse to the formation of 

 the soredia; but in the Pertusariaceae 3 , a family of crustaceous lichens, there 

 has been evolved a type of endogenous soralium which originates with the 

 medullary hyphae. In these, special hyphae rise from a weft of filaments 

 situated just above the lowest layer of the thallus at the base of the medulla, 

 the weft being distinguished from the surrounding tissue by staining blue 

 with iodine. A loose strand of hyphae staining the usual yellow colour rises 

 from the surface of the " blue " weft and, traversing the medullary tissue, 

 surrounds the gonidia on the under side of the gonidial zone. The hyphae 

 continue to grow upward, pushing aside both the upper gonidial zone and 

 the cortex, and carrying with them the algal cells first encountered. When 

 the summit is reached, there follows a very active growth of both gonidia 

 and hyphae. Each separate soredium so produced consists finally of five to 

 ten algal cells surrounded by hyphae and measures S/JL to 13/x in diameter. 

 The cortex forms a well-defined wall or margin round the mass of soredia. 



A slightly different development is found in Lecanora tartarea, one of 

 the "crottle" lichens, which, has been placed by Darbishire in Pertu- 

 sariaceae. The hyphae destined to form soredia also start from the weft of 

 tissue at the base of the thallus, but they simply grow through the gonidial 

 zone instead of pushing it aside. 



In his examination of Pertusariaceae Darbishire found that the apothecia 

 also originated from a similar deeply seated blue-staining tissue, and he con- 

 cluded that the soralia represented abortive apothecia and really corresponded 

 to Acharius's "apothecia of the second order." .His conclusion as to the 

 homology of these two organs is disputed by Bitter 4 , who considers that 

 the common point of origin is explained by the equal demand of the hyphae 

 in both cases for special nutrition, and by the need of mechanical support 

 at the base to enable the hyphae to reach the surface and to thrust back the 

 cortex without deviating from their upward course through the tissues. 



C. DISPERSAL AND GERMINATION OF SOREDIA 



Soredia become free by the breaking down of the hyphal stalks at the 

 septa or otherwise. They are widely dispersed by wind or water and soon 

 make their appearance on any suitable exposed soil. Krabbe 6 has stated 



1 Schwendener 1860. 2 Nilson 1903. 3 Darbishire 1897. 4 Bitter 1901, p. 191. 



5 Krabbe 1801. 



10 2 



