MORPHOLOGY AND PHYSIOLOGY. 41 



1. The Crustose Type. 



This thallus occurs in the lower lichens, that is, in 

 those in which the evolutionary specializations as 

 lichens are not yet highly marked. In its simplest 

 form it consists merely of a network of hyphae in 

 which are suspended a few algas ; it may occur on the 

 substratum, or wholly, or partially, beneath its sur- 

 face. There are no distinct layers, in fact, in many 

 instances it is almost impossible to detect any thallus 

 whatever. Such rudimentary thalli occur in the lower 

 species of Calicium^ Pyrenula, Trypethelium, Xylo- 

 grapha, Arthonia and other genera. 



The higher crustose thalli are quite thick, as in Le- 

 canora, Pertusaria, the southern representatives of 

 Gh'apkis, in Pceomyces, and other genera. The sur- 

 face is often warty or the entire thallus is marked off 

 into many-sided areas or areoles and is therefore spoken 

 of as areolate. In higher crustose thalli we find the 

 tissue-elements arranged into distinct layers, as seen 

 in a vertical section. The upper layer consists of 

 hyphae and constitutes a protective covering for the 

 layer of algae just beneath. The algje are inclosed by 

 specialized branches of hyphse, spoken of as haustoria. 

 Below the algae occurs another layer of hyphae which 

 attach the thallus to the substratum and assists in tak- 

 ing up soluble food-substances. The hyphae forming 

 the upper layer, the haustoria, and the lower layer are 

 continuous, and are part of the same hyphal tissue. 

 In many instances the upper layer becomes more or 

 less distinctly cortical, thus resembling the ui)per 

 layer of foliose thalli. In fact, very often it 



