93 



soluble food-substances, as well as for the purpose of forming a firm 

 attachment ; about the same time the portion opposite the substra- 

 tum undergoes considerable change; the hyphal cells forming the 

 soredial covering elongate, the enclosed algae multiply by division 

 and are carried upward by the elongating cells to which they are 

 adherent. In all respects it resembles apical growth in the lichen- 

 thallus : it is in fact the beginning of the thallus, from which in time 

 the various tissue layers are differentiated. In the formation of 

 all soredia the hyphae precede and direct the development of the 

 enclosed algae. The plate illustrating the formation of the ectotro- 

 phic cephalodia of PeUigcra aphthosa will also serve to illustrate the 

 growth of a soredium. 



Each soredium is capable of developing into a mature lichen bear- 

 ing perhaps apothecia and soredia, as well as the other lichen-struc- 

 tures. Sometime," two or more soredia unite to form one thallus, 

 this happening quite frequently among the Parmelias and other foliose 

 species. As already indicated, the soredia may develop upon the 

 mother-plant, especially upon the older and dying portions, where 

 they produce the peculiar isidioid outgrowths, especially noticeable 

 among foliose lichens. The numerous sterile vegetative branches of 

 Usnea are, perhaps, also soredial products. 



As to the relative value of spores and soredia as organs of repro- 

 duction, there can be little dispute. The soredia are by far the more 

 important ; many lichens are almost entirely dependent upon the 

 soredia for propagation, as is evident from the fact that no spores are 

 ever developed. It is also probable that lichens, which produce 

 spores quite constantly, are regularly propagated by means of the 

 soredia. From the very nature of lichens, spores can play only an 

 insignificant part in the maintenance of the species. 



There is another form of multiplication which is no doubt of less 

 frequent occurrence than propagation by means of the soredia, but 

 still more important than reproduction by means of the spores, and 

 that is typical vegetative propagation, which will now receive brief 

 mention. 



III. VEGETATIVE PROPAGATION. 



Any portion of the lichen-thallus may become detached and de- 

 velop into a new lichen, provided the fragment contains both symbi- 

 onts. Even the entire thallus may be torn away and carried to 



