1905] SNOW—DEVELOPMENT OF ROOT HAIRS 19 
There seems, therefore, to be a decided shortening of the cells 
in the substratum. As the mud at the bottom of the aquarium was 
of closer texture than the quartz, it probably offered greater resist- 
ance to the growth of the root. If the statement made by HaBeEr- 
LANDT (22, p. 188) concerning Elodea—‘‘denen die Wurzelhaare 
in Wasser vollkommen fehlen, wahrend sie beim Eindringen der 
Wurzeln in Erde sich reichlich einstellen”-—means that the resist- 
ance of the substratum is instrumental in bringing about the pro- 
duction of root hairs, these results agree. The possibility of the 
chemical stimulus of the soil has been mentioned. 
It does not seem probable that surface contact is a factor in the 
production of root hairs in soil, for when the earth is saturated the 
hairs on corn seedlings disappear, and those on wheat seedlings 
are decreased, although the soil particles are still there. This was 
stated by SCHWARZ (75, p. 160). In order to test the effect of con- 
tact with a smooth, solid body upon the epidermal cells of the root, 
corn seedlings were grown with their roots between glass plates, 
and in glass tubes open at the end. Where the roots filled the diam- 
eter of the tubes or the space between the plates, hairs were absent, 
both in air and water. On the sides not touching the plates hairs 
appeared nearly to the tip in air, and in the upper portions in water, 
as they do under ordinary conditions. Thus the contact on two sides 
of the root has no apparent effect on the hair production on the 
other two sides. Where the root did not fill the tube, hairs appeared 
in the usual zone in water and bent against the glass. 
RETARDATION OF GROWTH. 
It is of importance when speaking of the effect of growth upon 
the production of root hairs to indicate the effective stage. When 
the statement is made that slowing the growth of a root favors the 
production of root hairs (§1, 52, 11), the retardation may be due 
either to fewer cell-divisions or to less elongation of the cells. 
A. Rate of growth in air and water. 
According to MER (51, 52, 53, p- 1279), retardation of the growth 
of a root produces or increases hair development. Thus lentil roots 
(52, pp. 665-6), growing straight and smooth in air, became pilif- 
erous when their growth was checked by the earth. Also, when 
