INTRODUCTION 5 



genera of algae (Cyanophyceae and Chlorophyceae) are repre- 

 sented. Though the alga within the thallus is sometimes 

 abnormal as compared with its condition when living free, usually 

 no malformation occurs. The thallus does not develop in nature 

 when the proper alga is lacking, and attempts in the laboratory 

 to induce the formation of normal thalli in the total absence of 

 algae have met with only indifferent success. It is evident that 

 ordinarily at least the fungous hyphae develop at the expense 

 of the alga, and there seems to be httle basis for the belief that 

 a mutually advantageous food relation exists. In fact the 

 phenomenon appears to be clearly of the nature of parasitism, 

 rather than of symbiosis. This fact has led many workers to 

 feel that the lichens are merely fungi, which have become special- 

 ized in their parasitism on algae. Some students who hold this 

 point of view favor interpolation of the lichens among the fungi. 

 Others, with an interest or knowledge more pronounced in one 

 group than in the other, prefer, for the sake of convenience, to 

 keep the two apart. Still other workers believe that the peculiar 

 association of fungus and alga existing here indicates that the 

 group is an ancient one with a more or less independent origin. 

 The older belief that the lichen is a single organism, of which 

 the green cells are an .integral part, has now largely disappeared. 

 A good general presentation of the present day situation is given 

 by Smith (1921). 



In the majority of lichens the fruitbodies are ascomycetous 

 in type, some being discomycetous and others pyrenomycetous. 

 A few lichens are basidiomycetous. Since none are phycomy- 

 cetous the group has relatively little interest in connection with a 

 consideration of the lower fungi. 



Myxothallophyta. — At the lowermost limit of the Thallophyta 

 there exist several groups of very primitive organisms which may 

 be conveniently discussed under the common heading of Myxo- 

 thallophyta. This name, first proposed in Engler und Prantl's 

 Die NatiirUche Pflanzenfamilien, means slime thallophytes 

 and alludes to the slimy aspect of the thallus, which is composed 

 of naked protoplasm. In this group are included the Acrasieae, 

 Lab5rrinthuleae, and Myxomycetes. Though clearly not closely 

 related, these forms agree in that the thallus is naked and more 

 or less amoeboid throughout at least a portion of its existence. 

 In this respect they show a pronounced similarity to some of 

 the simplest forms of the animal world. In fact many zoologists 



