442 



the number of branchings. In cross section the podetium is seen 

 to have a radial structure, hollow, bearing a gonidial zone just 

 within the outermost protective (cortical) covering. The assimi- 

 lative surface of the podetium is usually much greater than that 

 of the thallus-lobe, from which it took its origin. Both Krabbe 

 and Schwendener* look upon the two distinctive forms of thallus 

 as a species of alternation of generation. Wainio "f believes that 

 the apothecial stalk is converted into a vertical thallus by a form 

 of metamorphosis. A similar view is held by Reinke ; that is, the 

 podetium was originally an apothecial stalk (perhaps comparable 

 to that of the Caliciaceae and of Baeoniyces) which finally became 

 metamorphosed into a true thallus. This metamorphosis was of 

 such a nature that the increase in size and assimilative function of 

 the podetium corresponded to a decrease in size and function of 

 the primary thallus. 



The author enters into a more detailed discussion of the mor- 

 phological characteristics of the leading types of Cladonia for 

 the purpose of illustrating the structural modifications of the 

 podetium and its relation to the primary thallus and apothecia. 

 Life-size figures of the types accompany this discussion. Reinke 

 quite frequently speaks of " fertile forms" when only " pycnidia" 

 are present, which shows conclusively that he considers these 

 structures as reproductive organs coequal in importance to the 

 apothecia. The fact that the pycnidia (and spermagonia) may 

 occur on either the primary thallus or the podetium, while the 

 apothecia occur on the podetia (excepting a few Cladonias which 

 are said to have no podetia or only pseudo-podetia) does not seem 

 to raise any question in the author's mind as to the feasibility of 

 considering the former as true sexual organs. This is only in 

 passing, but it is well to remember that sexual organs are gener- 

 ally not so variable in their occurrence and position on the vege- 

 tative portion as the pycnidia and spermagonia evidently are. 



Without going into further details the following summary may 

 be given of Reinke's reasons for assuming that the podetium is a 

 thallus rather than a reproductive organ. 



* Schwendener, S. Untersuchungen iiber den Flechtenthallus. Nageli's BeitrSge 

 zur Wissenschaftlichen Botanik, 2 : 169. i860. 



f Wainio, Monographia Cladoniarum Universalis. Helsingfors, 1867. 



