68 



more algae are thus enclosed, and in the majority of cases the alga or 

 algae which are being enclosed by the hyphal branches divide so that 

 the mature soredium contains a considerable number of them. The 

 cell-walls of the enclosing hyphal tissue become considerably gela- 

 tinized and closely agglutinated, thus forming a tissue especially 

 adapted to prevent the evaporation of moisture. Each soredium is 

 pushed outward by the elongating filament to which it is attached 

 and others are formed below it ; they are all loosely held together 

 by the united hyphal filaments of the thallus ; those uppermost are 

 most loosely connected and are readily torn away by air currents. 

 The distribution of soredia is analogous to the distribution of the 

 pollen of anemophilous flowers. If the soredia fall upon suitable 

 places they will begin to develop at once ; if not they may lie dor- 

 mant until the conditions are favorable for their development or they 

 may be carried by the wind to some more suitable locality. 



Each soredium may also develop other soredia producing the sp- 

 redial dust which sometimes covers considerable areas ; this happens 

 where the surroundings are not favorable to the development of 

 a thallus and yet not sufficiently unfavorable to cause the cessation 

 of all growth. 



Reference should also be made to the isidioid or warty soredial 

 branches which occur on Usnea and on many foliose thalli ; they are 

 soredia which have developed into imperfect thalli upon the mother 

 thallus, and are known as secondary thalli or phylloclades (Reinke). 

 There is, however, no doubt that in many cases these secondary 

 branches, which are comparable to leaves, are not developed from 

 soredia, but are a direct product of the algal zone of the mother 

 branch. 



4. THE THECIAL ALGAE (HYMENIAL GONIDIA) . 



Among various groups of lichens, especially the Verrucariaceae, 

 algae are found which regularly occur upon and about the constitu- 

 ents of the thecium ; this is a constant occurrence in Dermatocarfon 

 (Endocarfion) fusillum. In this case the algae are small and belong 

 to the genus Pleurococcus (see Dermatocarfon). Fiiisting, Stahl 

 and others state that these thecial algae are derived from the algae 

 of the thallus. Due to a change in nutrition, the alga-cell of the 

 thallus, on entering the apothecium, divides into a number of smaller 

 'cells, which adhere to the gelatinized paraphyses and to the walls 



