Mar., 192 1 ] MCDOUGALL — ROOT HAIRS OF GLEDITSIA 
Illinois), and the roots were examined to a depth of four feet. Thick- 
walled root hairs were found as abundantly at that depth as near the surface. 
In some cases the smaller parts of the root system are completely covered 
with root hairs while in other cases there are patches which are entirely free 
from hairs. These latter are probably due to the fact that the movements 
of the roots in the soil due to growth often break off the hairs. 
Early in February, 1920, some seeds of Gleditsia triacanthos were ob- 
tained and planted in the greenhouse. After seedlings had appeared, the 
root systems of some of them were examined at the end of one week, three 
weeks, four weeks, eight weeks, fourteen weeks, and twenty-one weeks. 
The following excerpts from my notes will serve as a report on this part of 
the work: 
Feb. 17. Seedlings one week old. The root (tap root) is 7 cm. long and unbranched. 
It is abundantly covered with root hairs throughout its entire length. Near the tip these 
hairs are whitish or colorless and have very thin walls. They contain protoplasm and look 
very much like the root hairs of other plants. Toward the base of the root the hairs are 
becoming brown and the walls are somewhat thickened. 
March 2. Seedlings three weeks old. The total height of the plant above ground is 
about 14 cm. The tap root is from 10 to 12 cm. long and has 10 to 15 branches varying 
from I mm. to 2 cm. in length. The entire root system is clothed with root hairs. The 
thickness of the walls of the oldest and brownest hairs is now about 0.95 n, while the walls 
of the young hairs near the tips of the roots are only about 0.5 ix thick. 
March 9. Seedlings four weeks old. The longest secondary root found at the age of 
three weeks was 2 cm. The longest at four weeks is 5 cm. These secondary roots, there- 
fore, may grow as much as 3 cm. in a week, or more than 4 mm. per day. The youngest 
hairs are usually found about 4 to 5 mm. from the tip, while at 12 mm. from the tip the 
hairs are beginning to turn brown. Therefore, they probably remain colorless only about 
two days. 
April 6. Seedlings eight weeks old. The thickness of the walls in the oldest hairs is 
now about 1.5 //. 
May 18. Seedlings fourteen weeks old. The walls of the oldest hairs have now reached 
the full thickness of approximately 2.0 ^t. The epidermis has ruptured in several places 
on the older parts of the root system, and the root hairs have disappeared from parts, 
July 6. Seedlings twenty-one weeks old. The entire length of the root system is now 
about 27 cm., and branches are numerous. Root hairs are present on most of the root sys- 
tem. They are very abundant as much as 15 cm. from the tips of some of the roots. On 
the first 7 or 8 cm. below the surface of the soil the epidermis has ruptured and many of the 
hairs have disappeared. Many still remain, however, even in this region, though they are 
often broken. There are other places on the root system where root hairs are entirely 
lacking, probably because of movements of the roots within the soil which break them ofif. 
It is apparent that in general the hairs remain for as long a time as the epidermis. 
During the spring and summer of 1920 the roots of a number of trees 
of Gymnocladus dioica and Cercis canadensis, both of which are closely 
related to Gleditsia, were examined. It was found that both of these 
trees produce thick-walled root hairs similar to those of Gleditsia triacanthos 
but by no means so abundantly as in the latter species. In Cercis they 
are even less frequent and less evident than in Gymnocladus. 
