9 2 



lichens make it seem probable that such is the case, but further care- 

 ful observations are, however, necessary to give a satisfactory solu- 

 tion of this problem. 



Germinating spores no doubt frequently enter into a symbiotic 

 association with the soredia which are likewise promiscuously dis- 

 tributed by the wind. In these cases it would also be interesting to 

 know whether the promycelium of Physcia stellaris, for example, 

 can enter into a symbiotic association with the alga or algae of a 

 soredium derived from a species of Parmelia or any other soredium 

 in which the algae are Cystococcus humicola. 



From the foregoing we may safely conclude that the spores of 

 lichens are not perfect organs of reproduction, and that they are 

 very unreliable as aids in reproduction. They can be looked upon 

 only as the degenerate reproductive organs of their fungal an- 

 cestors. If this be true there must be some evidence of such de- 

 generation. This evidence we actually find, especially in the higher 

 phyletic series. For example, in the more highly developed Par- 

 melias the spores are few or wholly wanting, as likewise in the 

 higher Cladonias ; in Thamnolia spores are always wanting. Some 

 general signs of spore-degeneration besides their less frequent oc- 

 currence or total absence are decrease or absence of coloring sub- 

 stances in the spore-walls, indistinct septa, thin spore-walls ; simple 

 colorless thin-walled spores occur also in the lower lichens but are 

 much less common than in the higher types. 



II. THE SOREDIA AS PROPAGATIVE ORGANS. 



The origin and morphology of soredia has already been dis- 

 cussed, and we shall here confine ourselves to a consideration of 

 their special importance as reproductive organs. They are phyloge- 

 netically derived lichen-structures and are in a certain sense the 

 typical reproductive organs of lichens. Morphologically it is, how- 

 ever, more correct to designate them as symbiotic propagative organs, 

 functionally comparable to the vegetative propagative organs of 

 higher plants. They are the only specialized propagative organs- 

 of lichens capable of developing at once into a new plant. 



The mode of propagation by means of the soredia is very simple. 

 As soon as a soredium is carried to some suitable locality develop- 

 ment begins ; the surface nearest the substratum sends out hyphal 

 branches which enter the substratum for the purpose of taking up 



