14 THE LOWER FUNGI— PHYCOMYCETES 



taining saprophytic forms, constitutes another illustration of 

 the point. Such border line cases make difficult precise gener- 

 alizations concerning the limits of the fungi. 



The reproductive processes of most of the fungi are far more 

 highly developed than those of any of the bacteria or slime thallo- 

 phytes. They are approximated in complexity in some of the algae. 

 Many of the fungi, apparently the majority, are polymorphic, 

 i.e., form two or more kinds of spores in the life cycle. These 

 different spore forms are designated either as perfect or imperfect. 

 The perfect spore form occurs more or less definitely in connection 

 with the sexual process, and is often called the sexual spore. 

 The imperfect spore form is asexual. More than one type of 

 asexual spore may be developed in a single species. The sexual 

 and asexual spores are often borne together or in succession on 

 the same thallus, but in some cases occur on different thalli. 

 Unless borne together the genetic connection between the two 

 may remain for a long time unsuspected. The phase of the life 

 cycle in which the perfect spore form occurs is termed the perfect 

 or sexual stage. That in which the imperfect spore form is 

 developed is called the imperfect or asexual stage. In both 

 cases, especially in the higher fungi, the spores are often borne 

 in definite aggregations or conceptacles termed in general 

 fruit bodies. 



In as far as possible, the classification of the fungi is based on 

 the characters of the perfect stage. In practically all modern 

 systems of classification the fungi are subdivided on this basis 

 into three major natural groups, — Phycomycetes, Ascomycetes, 

 and Basidiomycetes. These subdivisions usually rank as 

 classes, are coordinate, are treated as units in the phylogenetic 

 arrangement, and are separated rather sharply from one another. 

 In these respects they differ essentially from the group Fungi 

 Imperfecti, which is usually appended to them as a fourth major 

 subdivision of the fungi. This group was erected for the temporary 

 accommodation of the imperfect stages of fungi which have not 

 yet been identified in connection with their perfect stages. There 

 are many hundreds of them. They are believed to be, in most 

 cases, imperfect stages of Ascomycetes. A smaller number 

 probably belong with Phycomycetes or Basidiomycetes. The 

 perfect stages of some of them may no longer occur in nature, or 

 possibly may never have been developed. The group as a whole 

 has no unity and consequently no proper place in a natural 



