6 7 



It is very difficult to draw a dividing line between the two kinds ; 

 all gradations being found between the simplest fungal type and the 

 most highly developed thalline type, nor is the matter simplified 

 by further subdividing the apothecia as Nylander and others have 

 done. I have designated as thalline all apothecia in which there is 

 unmistakable evidence that the algae enter into the formation of the 

 exciple ; this leaves a large number of apothecia which must be classi- 

 fied with the fungal type in which the cortical and medullary layers 

 of the thallus aid in forming the exciple. 



I have purposely avoided employing technical terms referring to 

 the different parts of the exciple, such as -pars marginalis, inargo 

 ■firoprius, pars thallina, exijmlum zeorinum as well as the terms re- 

 ferring to apothecial type-variations. It is hoped that the description 

 of the typical apothecium and the comparison of the fungal and 

 thalline types will make clear to the student the variation of the 

 apothecium in its phylogenetic development. 



3. THE SOREDIA. 



Structurally, the soredium proper is a spherical body consisting of 

 from one to many algae surrounded by a continuous hyphal tissue ; 

 functionally they are the chief vegetative propagative organs of 

 lichens. They are in a certain sense the typical reproductive organs, 

 since the spores, in themselves, are wholly incapable of developing 

 into a new lichen. Each soredium contains both symbionts in a suit- 

 able relationship for developing into a new lichen as soon as favor- 

 able conditions present themselves ; they might be compared to 

 miniature lichen-thalli, as will be evident from a study of their mor- 

 phology as well as their development. 



Soredia are more or less common upon the upper surface and 

 margin of most of the higher lichen-thalli. They take their origin 

 from the algal layer at points where the upper cortical layer is 

 broken ; they are sometimes quite uniformly distributed over the 

 entire surface of the thallus, giving it the appearance of being cov- 

 ered by pollen dust ; they occur very rarely upon the lower sur- 

 face. The soredia are formed as follows : A hypha of the algal 

 layer still adherent to an alga, if it occurs near the above mentioned 

 breaks in the cortical layer, begins to form additional branches or 

 haustoria which enclose the alga ; in time the alga is entirely 

 enclosed by this hyphal tissue : more frequently a group of two or 



