244 STRUCTUKAL BOTANY 



remained undisturbed, the Lichens were rightly ranked 

 as a distinct Class, equivalent to Algae and Fungi, and in 

 some respects intermediate between them. Of late 

 years, however, evidence has accumulated, which proves 

 conclusively that the gonidia do not belong to the 

 same plant with the hyphae, but that they are distinct 

 organisms, identical with definite genera and species of 

 the Algae. Hence a Lichen is in reality a compound 

 organism, made up of two totally different plants, an 

 Alga and a Fungus, living in the closest association, 

 and mutually dependent one on the other for certain 

 essential services. 



The evidence on which this striking conclusion is 

 based is of various kinds. In the first place, the so- 

 called " gonidia " of Lichens are always found to agree 

 exactly with certain species of the lower Algae, which are 

 also well known in a free and independent condition. 

 Thus in our type Physcia parietina, the algal constituent 

 is Cystococcus humicola, a unicellular form nearly related 

 to Pleurococcus. Further, the " gonidia " have been 

 isolated from the Lichen-thallus, and are then able to 

 lead a perfectly independent life, growing and multiplying 

 on their own account, just like their fellows which have 

 never been in bondage. More recently it has been found 

 possible to raise a Lichen, that is to say the fungal 

 constituent of a Lichen, by growing its spores in a food 

 solution, which, as Algae are absent, must of course contain 

 organic food substances. - In this way a small Lichen- 

 thallus can be produced, but it never contains any gonidia. 

 Thus the fungal as well as the algal constituent can, 

 under suitable conditions, live by itself. 



Most conclusive of all, however, is the actual synthesis 

 of a Lichen, that is, the building up of a new plant out 



