146 



PRACTICAL BOTANY. 



have grown towards each other on what was the outer 

 end of the soredium, as it grew 011 the parent Lichen, 

 forming an arch over the algal portions. The soredia 

 separate from the Lichens as they dry after rain, and are 

 then blown about by the wind. This mode of distribution 

 of the soredia and spores explains the abundance and wide 

 distribution of Lichen growths. (See Fig. 84). 



FIG. 84. A, B, C, soredia of Usnea barbata in their earliest stages; D, E, 

 the same developing arched growths of the fungal constituent; a, b, c, 

 soredia of Physcia parietina in different stages of development. (From 

 Goebel, after Scliwendener.) 



The arching growth of the hyphae over the Algae at 

 the outer end of the soredium also illustrates the typical 

 method of growth in much the largest number of heter- 

 omerous Lichens. The Fungus grows in advance, and 

 leads the way for the Algae. In a few heteromerous 

 Lichens the Algae grow in advance of the Fungi. In the 

 homoiomerous Lichens it has not yet been determined 

 whether the Fungus or Alga grows in advance. 



References for Reading. Goebel's "Outlines of Classification," 

 pp. 114-126; Sachs' "Physiology of Plants," pp. 391, 514; Vines' 

 " Text-Book of Botany," pp. 319-320 ; Bennett and Murray's " Crypto- 

 gamic Botany," pp. 318-322 ; Bower's " Practical Botany," pp. 473- 

 480 ; Bessey's " Essentials of Botany," pp. 165-169 ; Strasburger and 

 Hillhouse's "Practical Botany," pp. 202-270; Carpenter's "The 

 Microscope," pp. 576-579 ; Geddes' " Chapters in Modern Botany," 

 pp. 114-119. 



