78 Plant Life and Evolution 



upon species of Juniper. Upon these cedar apples 

 in the spring are produced great masses of orange- 

 yellow spores imbedded in a soft jelly. From these 

 spores arise others which will not grow upon the 

 cedar, but will germinate if they are carried to the 

 opening leaves of a thorn or crab apple, upon which 

 they produce a fungus growth entirely different 

 in appearance from that upon the cedar. In little 

 cup-shaped receptacles which appear later upon the 

 leaves of the thorn, are borne chains of spores, 

 which, carried back to the cedar, give rise to a new 

 crop of cedar apples. What is the meaning of this 

 change of host is not clear, but it is paralleled by the 

 behavior of many animal parasites like Trichina and 

 the liver-flukes. 



Symbiosis. Many fungi live in a more or less 

 perfect symbiotic relation with other plants. The 

 best-known cases are those of the Lichens, which, as 

 is well known to the botanist, are associations of 

 fungi, usually sac-fungi, with various low algae. 

 If we examine the structure of a lichen, it is easy 

 to see that it is much like a true fungus, but en- 

 meshed among the colorless filaments of the fungus 

 are colonies of green cells, which a close examina- 

 tion shows to be unicellular algae, upon which the 

 fungus filaments are parasitic. It is these green 

 cells, imprisoned in the tangle of fungus filaments, 

 which give the greenish tinge to the lichen. Ex- 

 actly what role each of the symbionts plays is not 

 entirely clear. The fungus is undoubtedly to some 



