88 ILLINOIS STATE ACADEMY OF SCIENCE 



character and that further study will reveal other char- 

 acters that may be used to greater advantage. It should be 

 said too that the characters as used apply primarily to the 

 superficial layers of soil to a depth of about one foot since 

 the mycorrhizal characters would not apply, as a rule, to 

 the deeper roots. 



The characters of which most use is made are the pres- 

 ence or absence of mycorrhizal structures, the colors of the 

 root bark and the relative size of the ultimate or smallest 

 branches. The appearances of the various types of mycor- 

 rhizas has been described in a previous paper.i Although 

 living ectotrophic mycorrhizas are usually absent from all 

 trees in late spring and early summer, yet in case of trees 

 which habitually produce mycorrhizas there is nearly al- 

 ways abundant evidence of them in the dead coral-clusters 

 of mycorrhizal roots. The color of the bark that is taken is 

 the color just within the surface after the dirt and the out- 

 ermost surface layer have been scraped away by means of 

 a blunt instrument such as the edge of a garden trowel. 

 The size of the smallest branches of the root systems of 

 various plants varies greatly in different species and is rea- 

 sonably constant. The smallest roots of some trees are very 

 coarse, those of others very fine and those of a third group 

 are intermediate in this respect. An intermediate size of 

 roots does not make a good key character but it has been 

 possible in the following brief key to use this character only 

 in those cases in which the roots are either conspicuously 

 coarse or fine. 



Other characters that are used in the key are the colors 

 of ectotrophic mycorrhizas, the odor of the roots, the pres- 

 ence or absence of endotrophic mycorrhizal "beads" and 

 the presence or absence of stiff brown root hairs. The first 

 two of these characters are obvious without any explana- 

 tion while the endotrophic mycorrhizal beads and the thick- 

 walled root hairs have been described in earlier papers.^ 



1. McDougall, W. B. — On the Mycorrhizas of Forest Trees. Am. Jour. Bot. 

 1 :51-74. 1914. 



2. McDougall, W. B. — loc. cit. and Thick-walled root hairs of Gleditsia and 

 related genera. Amer. Jour. Bot. 8:171-175. 1921. 



