1905] SNOW—DEVELOPMENT OF ROOT .HAIRS 35 
complete extraction of oxygen than in former experiments, than to 
obtain quantitative results. The oxygen pressure was considered 
approximately one-half, and the roots indicated about the limit of 
hair production, showing irregular patches and scattered hairs with 
bare spaces. Repetition of the experiment showed the same condition 
of hair production. The 
temperature varied from 
20-24°, which was proba- 
bly not sufficiently high 
or low to effect hair pro- 
duction. 
Willow twigs set up in 
Wolf’s flasks in the same 
manner in about half 
oxygen content, with their —— = = 
lower ends in water, after Fic. 15.—Diagram showing apparatus for 
three days showed hairs diminished oxygen pressure; the air passes through 
two jars of pyrogallic acid solution before entering 
the experiment jar. 
on the laterals in both 
jars. Inseven days there 
was decidedly less air in the partial pressure jar. Left about two weeks 
longer, the hairs were better in both jars, appearing better in water 
than inair. This may be on account of accommodation to lack of 
oxygen (PFEFFER 64, p. 2), or more probably to an increase in the 
supply by the green bark and the chlorophyll appearing in the roots. 
GENERAL CONSIDERATIONS. 
Many writers (KRAEMER 34; LEAVITT 40, 41; VAN TIrcGHEM 78; 
JurL 27a; SAUVAGEAU 72, p. 5 for Naias and possibly for other 
forms, '73, p. 169) associate short cells with root hairs, in most of the 
cases mentioned the cells being preformed. From many measure- 
ments of sections cut from roots grown in these experiments there 
-appeared to be a relation between the length of the cells and the 
growth of hairs, but there was no evidence of the preformation of the 
piliferous cells. No definite length of cell can be given as the limit 
for hair development, either in general or in a single species; the 
piliferous cells of one root may be longer than the smooth cells of 
another root of the same species. But an average derived from many 
