172 AMERICAN JOURNAL OF BOTANY [Vol. 8 
these roots presented the appearance that is seen in figures i and 2 : bristUng 
with projections which I still thought to be due to a fungus. A cross section 
examined under the microscope, however, showed at once that these 
projections are thick- walled root hairs; figure 3 being a camera lucida draw- 
ing of one of these hairs. The actual thickness of the walls of these hairs, 
as measured with a Spencer Lens Co. screw micrometer, averages approxi- 
mately 2 Id. This is about four times as thick as the walls of the root hairs 
of barley seedlings grown in petri dishes on filter paper. It is also about 
four times as thick as the walls of the newly formed hairs on the roots of 
honey locust seedlings grown in soil, as will be brought out later. 
The hairs do not average very long, only about four tenths of a milli- 
meter, but they are stiff and rigid and do not shrivel when exposed to the 
air. The photographs shown in figures i and 2 were taken from specimens 
that had lain unprotected on my desk for more than a month, during which 
time they had not shriveled in the least. 
The chemical composition of the walls has not been determined. It is 
certain that chemical changes take place during the thickening of the walls, 
Fig. 3. A thick-walled root hair of Gleditsia triacanthos. 
but repeated tests with phloroglucin and hydrochloric acid failed to detect 
any lignification while a cellulose reaction was finally obtained by allowing 
sections to remain in chloriodide of zinc solution for twenty-four hours. 
It has also been impossible to' determine whether these hairs contain living 
protoplasm, for the walls are so thick and so dark-colored that it is not 
possible to see through them. 
During the summer of 191 9 I examined the roots of a large number of 
Gleditsia triacanthos trees in various localities and in very diverse habitats, 
the habitats ranging from the climax upland forest of Champaign county, 
Illinois, to the typical bottomland forests along the Vermillion River in 
Vermillion County, Illinois. To my surprise I found that the presence of 
thick-walled root hairs is a constant character of this species. I have not 
been able to find a honey locust tree on whose roots the hairs were not present 
in abundance. In order to determine whether the hairs are present on 
more than the superficial roots, a hole was dug near a honey locust tree in 
the "University Woods" (a sixty-acre tract of climax forest belonging to 
the University of Illinois and situated four miles northeast of Urbana, 
