236 IMPORTANCE OF LICHENS 



The ascocarps derived from this reproductive process assume a 

 great variety of shapes (Fig. 142) but they all show the hymenial 

 layer of asci and paraphyses (Fig. 144) or of basidia and para- 

 physes in the few forms belonging to basidio-lichens. 



The ascospores germinate readily, producing hyphae which, 

 however, soon perish unless they chance to meet an alga with 

 which they can live. It should be noted that these fungi appear- 

 ing in lichens cannot associate indiscriminately with any species 

 of algae. Each species of fungus is adapted to one or more species 

 of algae and is unable to live with any other form. As may be 

 imagined, the real nature of the lichen was for a long time mis- 

 understood, the algae even being looked upon as spore bodies. 

 The bitter dispute over the question was finally settled when a 

 lichen was produced artificially by bringing together a suitable 

 fungus and alga. 



(c) Economic Value. Lichens are of considerable economic 

 importance in the world. Their service in hastening the decay 

 and transformation of rock material has already been referred to. 

 In many sections they form the characteristic vegetation of the 

 country, as in certain alpine and desert regions and in the north- 

 ern barrens, where they afford rich pasturage to reindeer and 

 caribou. Their abundant growth in mountainous districts and 

 subsequent distribution by winds and rains accounts for the 

 showers of manna in biblical history. Certain species are still 

 used in northern regions to lengthen out a meager food supply. 

 Litmus, employed in acid testing, and various pigments are 

 derived from lichens, as were also the famous purple and blue 

 dyes of the East. 



87. A Third Line Possibly Related to the Pezizales. The 

 following orders may not represent a single line of descent but 

 there are certain features in their life history indicating that 

 they may represent a line of departure from the Pezizales. 

 These forms are mostly minute; parasitic, or saprophytic upon 

 dead plants. The more epiphytic position of these forms may 

 have been a factor in the reduction in size. Certainly the 

 larger fleshy forms of the preceding orders are terrestrial in 

 habitat. There are two quite distinct groups in this alliance, 



