io6 



the apothecia and the spores, and, incidentally, to consider the sub- 

 stratum upon which the plants grow, and their distribution, both hori- 

 zontal and vertical. 



II. CHEMICAL REACTIONS. 



For some years the chemical behavior of lichens to certain re* 

 agents has been considered of great importance in delimiting 

 species. After careful testing I have finally decided to abandon 

 the use of these reagents since they are practically valueless for the 

 purpose recommended. That there are marked chemical reactions 

 cannot be denied, for example the blue iodine-reaction of the thecial 

 wall of the majority of lichens. This reaction is, however, so gen- 

 eral in its occurrence that it can not possibly be of any value in estab- 

 lishing species, and the coloration varies in different individuals of 

 the same species or even in the same plant at different periods of de- 

 velopment. The same may be said of the other reactions, as those 

 with solutions of potassium hydrate and calcium hypochlorite. That 

 these reactions should be unreliable is evident when we consider the 

 life-history of lichens ; different individuals of one and the same 

 species may develop upon substrata of widely different chemical 

 composition. These chemically different substances adhere to the 

 lichens and some soluble constituents are also taken up by the fungal 

 symbiont which accounts for the difference and unreliability of the 

 chemical reactions. It has already been indicated that perhaps 

 some of the chemical compounds detected in lichens are derived from 

 the substrata by absorbtion and are not the results of metabolic (or 

 catabolic) processes in the plant itself. 



The advocates of the chemical reactions advise great care in 

 selecting chemically pure reagents and caution against carelessness 

 in cleaning slides, cover glasses, etc. They advise a carefulness 

 which is not called for ; it is known that a rock-lichen and a tree- 

 lichen of the same species are exposed to different environments, no 

 matter how careful the experimenter may be in the selection of rea- 

 gents and in cleanliness there is a difference in the chemical com- 

 position of the two plants which will more than neutralize the care- 

 fulness of the investigator and manifest itself by a difference in re- 

 action. 



The blue reaction with solutions of iodine is sometimes also notice- 

 able in the hyphal tissues outside of the spore-sacs and paraphyses - r 



