42 BOTANICAL GAZETTE [yuLY 
' It would be well to limit the term “root hair” to hairs borne by | : 
morphological roots only. 
SUMMARY. 
1. Light and darkness appeared to have only an indirect effect, 
through their influence on growth. 
2. High temperature with sufficient moisture tended to decrease — 
hair production by increasing the elongation of the internal cells. 
3. The slower the growth in air the better the hair development. _ 
4. Retardation of growth by glass tubes, by wounding, or by 
resistance of the substratum favored hair production. 
5- Roots of seedling corn in water first curled and produced hair, 
possibly because of the retardation of growth by the diminution of a 
oxygen or its presence in the dissolved state. Later the roots grew 
straight and smooth, either on account of accommodation to the 
oxygen supply or because the gas was supplied by the aerial parts. 
6. Saturated air with high temperature tended to suppress hair 
development (cf. 2). 
7- Saturated soil tended to suppress hair in corn and wheat, 
but other factors must be considered when Elodea develops hair in 
the substratum. . 
8. Osmotic solutions gave very irregular results on account of 
some undiscovered disturbing factor. 
9. Less hair was developed in distilled water than in tap water. 
to. Air deprived of oxygen stopped hair production and retarded 
growth. 
11. Curves and swellings had a favorable effect upon hair develop- — 
ment, probably because they represent the retardation of the growth 
of the root. 
12. In all these examples of retardation favoring hair develop- 
ment, not the mere rate of growth, but the differential elongation 
of the inner and outer cells was of prime importance. Hair produc- 
tion depends on the ratio between the capacity of the epidermal 
cells to elongate and their ability to do so. 
13. The activity of the epidermis may be in inverse proportion | 
to the activity of the central cylinder, lateral roots often appearing — 
when hairs are suppressed, and vice versa. 
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