1 04 



in a work of this kind to enter into a discussion of the various systems 

 and to explain their deficiencies. The student is advised to consult 

 the various works in which the classifications mentioned are em- 

 ployed. 



Family Characters. In making the next comprehensive subdivi- 

 sions of the order ascolichenes there are several important charac- 

 ters to be considered ; such as the probable fungal ancestors, the 

 structure and development of the apothecia and of the thallus. It 

 must be admitted that the limitations of the families is as yet very un- 

 satisfactory ; this is in a great measure due to our uncertainty in re- 

 gard to the identity of the fungal ancestors. It is very probable that 

 the number of lichen-families could be made to correspond to the 

 groups of fungi from which the fungal ancestors were derived. Al- 

 though the development and structure of the apothecia are of great 

 importance in defining the families there are other less important 

 characters. For example, in the family Physciaceae the spores and 

 in the Collemaceae the algae. The variable exceptions simply indi- 

 cate that the present family limitations are far from reliable. 



Generic Characters. Theoretically, there are at least as many 

 genera as there are natural series derived from distinct prototypes. 

 This probability is especially applicable to the lower and smaller 

 series, that is, those natural groups in which the phylogenetic pro- 

 cesses have produced only comparatively slight changes. In the 

 larger and biologically (therefore, also, morphologically) more com- 

 plex series it is found convenient to recognize several or many 

 genera, and such genera may even be distributed into different 

 families. It is highly probable that Lccidea, Biatora (Lecidea- 

 ceae) and Parmelia (Parmeliaceae) belong to the same series phylo- 

 genetically derived from a single prototype. The fact that they are 

 not included in the same family is in part due to the uncertainty of 

 their relationship and in part because an arrangement under separate 

 families was found most convenient. 



The spore-characters are of prime importance in delimiting gen- 

 era. Constantly colored and colorless spores are not to be included 

 in the same genus, but it must be remembered that all spores are 

 colorless in the early stages of development. The number and 

 direction of the septa are of great importance ; in the majority of 

 spores the septa are formed in two planes, that is, either only at 

 right angles to the longitudinal axis of the spores or also in the longi- 



