10 THE CHICAGO ACADEMY OF SCIENCES. 



others. It encloses them as a spider its prey, with a network of 

 delicate tissue, which is gradually transformed into an impervious 

 integument. But while the spider sucks the life out of its prey, 

 the Fungus stimulates the algae in its grasp to greater activity, to 

 a more vigorous increase, and thereby renders possible a more lux- 

 urious growth, and promotes the welfare of the whole colony." * 



However grand the Schwendener theory, the question of auton- 

 omy is still open, while new discoveries are being made which may 

 eventually change the whole aspect of the science. Our best way 

 now is to effect their classification under the present system as elu- 

 cidated by Tuckerman, but not ignoring others entirely. 



The student seeking to become acquainted with lichens will 

 soon learn to distinguish those highest in the scale, as Parmelia and 

 Physcia, in our vicinity, by the prominent, foliaceous thallus, 

 and apothecium, and having fixed the external character in his 

 mind, will soon associate with them others of similar appearance, 

 yet differing to some extent in thallus and fruit. In the same man- 

 ner he will identify and bring together species of other genera, 

 and perhaps stumbling on forms of Lecanora and Pyrenula, lower 

 in the scale, observe the characters that seem prominent and com- 

 mon to all. Experience will enable him to determine very man}^ of 

 our common species from external characters alone, as indeed, the 

 early lichenologists determined them all, even up to the time of 

 Webber and Acharius. 



THE DIVISIONS OF LICHENS. 



All our lichens may be placed in one of the two series into 

 which they are divided by the fruit. First, the Gymnocarpi, those 

 with open fruit; second, the Angiocarpi, those with closed fruit 

 Pyrenocarpous). There are five tribes, Parmeliacei, Leoideacei, 

 Graphidacei, Caliciacei, Verrucariacei. Only the last belongs to the 

 second series. The tribes are based on the apothecia; families on 

 the thallus and fruit. Genera and species follow. Other sub- 

 divisions are made for convenience, and sometimes at the caprice 

 of authors. 



*.Die Algentypn FleclitevgonkUeu. 



