CHAP, ii.] CHEMISTRY, ETC., OF PLANT LIFE. 85 



large group of cryptogamic plants called Lichens, that 

 were until recently considered autonomous plants, as 

 their relations, the Algae and Fungi, are in reality. It 



Fig. 24. Illustrations of the life-cycle of the parasitic fungus known 

 as "rust" on wheat and other grasses (Puccinia graminis). I. the 

 rust -coloured streaks of the fungus on stem and leaf of wheat plant 

 (natural size) ; II. spore of the spring stage, known as uredospores 

 (highly magnified) ; III. two-celled spore of autumn stage, known as 

 teleutospores (highly magnified) ; IV. a teleutospore germinating and 

 producing a promycelium bearing three promycelium-spores (highly 

 magnified); V. a leaf of the barberry bearing groups of the "cluster- 

 cup " or JEcidium, stage (natural size) ; VI. a portion of a barberry leaf 

 with a group of "cluster-cups" (slightly magnified); VII. a single 

 spore belonging to the secidium stage, called an cecidiospore (highly 

 magnified). 



has, however, been clearly proved that every lichen is 

 what may be termed a compound organism, consisting 

 of a fungus and an alga leading a life of mutualism, and 



