THE LICHEN-FLORA OF CHICAGO AND VICINITY. 9 



like or coralloid. This condition gave rise to the pseudo-genus 

 Lepraria, Isidium, etc., not now recognized. Lichens may grow 

 for a century or more, and under poor surroundings increase by 

 bisection, Hke Algae. They will be found under these conditions 

 near towns and cities, where the air is not pure. 



The thallus and the apothecium, or fruit, are the parts of the 

 plant to first attract attention in the higher lichens — for example, 

 the Parmelias, so common on our oaks. The thallus is attached 

 to its substrate by fibrils, or is subcortical, but is absent in certain 

 species, as the parasitic ones. 



In the thallus are green cells called gonidia, and other or- 

 gans, as spermogonia and pycnidia. Dr. T. M. Fries constructed 

 a system based on the gonidia, which are variously formed and 

 colored. The thallus supports the apothecium, which is the 

 most important part of the plant. This appears on the 

 surface, disk-like, lirellate, etc., and of several colors, such 

 as red, orange, black or brown. In some genera the fruit 

 appears as minute, wart-like bodies, with only a pore (ostiole- 

 at the summit. In others, as Cladonia, the fruit is at the 

 end of thalline stalks (podetia), or as in Cetraria, on the edges 

 of the thallus. There are, however, various modifications of all 

 these. One peculiarity of some lichens is the power they have to 

 burrow into the hardest rocks, even flint and granite, thus making 

 for themselves homes (foveoli). The power is furnished by the car- 

 bonic acid absorbed from the air. As an example, Verrucaria ru- 

 derella, Nyl., from the limestones of La Salle count}'. 111., may be 

 mentioned. The apothecium contains the germinating cells, or 

 spores, which are inclosed in sacks, called thekes. There are 

 many organs, all of which must be examined under the microscope, 

 and these have been the subject of profound study and discussion 

 many years, but especially since the microscope came into use. On 

 them many fanciful theories as to the origin of lichens, whether they 

 are autonomous or not, have been built. Fries called them aerial 

 algae. Schwendener gives his views as follows, which seems 

 marvelous, if true: " All these plants are not individuals in 

 the common sense of the term; they are rather colonies, consisting 

 of hundreds of thousands of individuals, of which one holds the 

 mastership, while the others, in eternal captivity, prepare the nour- 

 ishment for themselves and their master. The master is a Fungus 

 of the class of Ascomycetae, a parasite which lives on the labor of 



