109 



the thickness of the thallus may amount to one-third, and the same 

 may be said of the different tissue-layers. The variation in the 

 size of the spores is also very great ; sometimes also the variation in 

 form. In consideration of these facts it does not seem justifiable to 

 establish genera based upon the relative thickness of the cortical 

 layers, as Schwendener has done. Since nearly all lichen-tissues 

 are highly hygroscopic, the dimensions of the various elements 

 will vary with the amount of moisture present. In order to make 

 the factors as constant as possible during examination, it is advisable 

 to have all sections in the same media ; if different media are used 

 it is necessary to make due allowance for the changes they produce 

 in the tissues. 



The histological characters are also somewhat variable, and this 

 is especially true of the hypothecium. In one section (of the same 

 plant) it may appear semicortical, in another hyphal and in a third it may 

 be almost impossible to recognize any structure, depending largely 

 upon the portion of the apothecium from which the sections are made, 

 and also upon the age of the plant. In general, however, the various 

 structures are constant in their morphological characteristics ; that is, 

 the various tissues, as upper cortical layer, algal layer, medullary 

 layer, lower cortical layer, and thalloid exciple, of different indi- 

 viduals of the same species are constant in occurrence and structure, 

 though variable in dimensions. 



Coloration of the tissues is quite variable, especially in the thal- 

 lus. This variation depends upon the substratum, the age of the 

 plant, the amount of the moisture present, the thickness of the vari- 

 ous tissue layers, etc. Most species soon lose their normal color 

 when placed in the herbarium, but if not too old the specimen maybe 

 made to regain its original color by immersing in water. Though 

 certain colors occur quite constantly, such as green, yellow (orange), 

 light-grey, black, greenish-blue, it will be found impossible to make 

 use of delicate shades and tints in the determination of species. 



Key to the Families. 

 Symbiotic algae belonging to the Chlorophyceae. 

 Thalloid exciple wanting. 



Stipes or podetia present. 



Spore-sacs dissolving, setting free the immature spores ; 



disk closed I. Caliciaceae. 



Spore-sacs not dissolving ; disk open, convex. 



II. Cladoniaceae. 



