Mar., 1921] MCDOUGALL — ROOT HAIRS OF GLEDITSIA 
mycorrhizas from the roots of these trees is probably to be explained by the 
fact that the walls of the root hairs become thick and hard so quickly 
that the parasitic organisms are unable to gain an entrance. It will be 
recalled that the three trees under consideration all belong to the subfamily 
Caesalpinioideae. The native species of Cassia, the only other genus of 
this subfamily represented in our flora, are all herbaceous. They produce 
root nodules, and so far as I have examined them they do not produce 
thick-walled root hairs. Rohinia Pseudo- Acacia, the common locust, belongs 
to the subfamily Papilionoideae. It also produces nodules and does not 
have thick- walled root hairs. 
Summary 
1. The root hairs of Gleditsia triacanthos become thick- walled and 
brown in color within a few days after they are produced. This takes 
place regularly in all sorts of habitats. 
2. These thick- walled root hairs persist as long as the root epidermis, 
whether this be for a few weeks or for several months, unless they are 
accidentally broken off by root movements in the soil. 
3. The root hairs of Gymnocladus dioica and Cercis canadensis some- 
times become thick-walled and brown, but this phenomenon is not nearly 
so characteristic of these two species as it is of Gleditsia triacanthos. 
4. The thick-walled root hairs are considered xerophytic structures 
and are believed to be relics of a time when the species which possess them 
grew only under xerophytic conditions. 
5. The trees which have thick- walled root hairs produce neither bac- 
terial nodules nor mycorrhizas. This is probably due to the inability of the 
parasitic organisms to enter the roots through root hairs with thickened 
walls. 
University of Illinois, 
Urbana, Illinois 
