Lecture II 



THE OCCURRENCE OF MYCORRHIZAE 



Reasons for Studying Mycorrhizal Occurrence: — The occur- 

 rence of mycorrhizae is a subject of the first importance in mycor- 

 rhizal study. It is important for two reasons : first, to determine the 

 extent of mycorrhizal occurrence, and, second, to determine mycor- 

 rhizal importance. It must be confessed that little is known of the 

 extent of mycorrhizal occurrence. As we examine the history of the 

 subject it is evident that human interest in mycorrhizae has followed 

 a usual pattern : first there has been a flurry of interest, then an 

 haphazard and eager collecting of various material from casual 

 sources, and finally a settling down to solving problems of isolated 

 detail that may or may not be important to the subject as a whole. 



It would seem to be more logical to examine first of all the plant 

 kingdom to determine whether mycorrhizae actually occur widely in 

 that kingdom. We assume that they do but the assumption is not 

 based on research. Two or three dozen investigators have gone into 

 the woods and fields, they have sunk their digging tools into the earth, 

 and whatever came up was made the subject of study. There are a 

 few good papers on mycorrhizal occurrence, but very few. The classic 

 paper is Janse's account of the mycorrhizae occurring about Buiten- 

 zorg in Java, and after half a century we still consult the paper. Janse 

 consciously limited his research to chosen representatives of various 

 plant families and as a preliminary study it is excellent, but it should 

 be followed by similar papers on other members of the same, and of 

 other, families. 



In consequence of the fortuitous method which has hitherto been 

 employed in mycorrhizal study, we have the following summary of 

 what is now on record : Considerable good work on some of the hepat- 

 ics ; nothing on mosses or arthrophytes ; a little on pteridophytes ; 

 good studies on the gametophyte generation of some lycopods ; a very 

 unequal emphasis on members of the Gymnospermae, with most mem- 

 bers unknown as to mycorrhizal condition (all attention must be de- 

 voted to a few pines and spruce!) ; and a scattering of information 

 about some angiosperms. On this slender evidence scientists assert 

 the importance of mycorrhizae. They may be important and probably 

 are important; but it is scarcely scientific to jump at conclusions. We 



