8 GUIDE TO THE STUDY OF LICHENS. 



observed the development of the lichen Collema limO' 

 sum from the alga Nostoc muscorum var. lichenoides. 

 A general belief of that time was that the so-called 

 " primal substance," or " green substance of Priestley " 

 (perhaps Protococcus vulgaris or other unicellular 

 alga) could develop into algae, lichens, mosses and 

 other lower plants. Nees von Esenback was wont to 

 guide his pupils to the old castle at Pottenstein in 

 order to demonstrate ad oculos how the green sub- 

 stance of Priestley occurring on the stone walls could 

 develop into a lichen, while if placed in water it would 

 develop into an alga. 



Thus we see that during this entire period, which 

 extended over two thousand years, scarcely anything 

 more was done than to name and roughly describe 

 between 500 and 600 species of lichens. The heated 

 discussion, the wrangling and the uncertainty in regard 

 to the origin and position of lichens was simply evi- 

 dence of ignorance and not a sign of profound knowl- 

 edge of the subject. Scarcely a ray of scientific light 

 had as yet penetrated the Stygean darkness in which 

 these highly interesting plants were enwrapped. It 

 simply shows that scientific progress, like all progress, 

 is slow and its devious path is strewn with thorns and 

 s tu mblin g-blocks . 



II. FROM WALLROTH AND MEYER (1825) TO THE 

 CLOSE OF 1896. 



This might be designated as the period of scientific 

 lichenology. Leading botanists now began to study 

 the life-history, the morphology, histology and physi- 



