18 BOTANICAL GAZETTE [JULY 
enough to make a layer, and consequently not enough to offer any 
appreciable resistance to root growth. The roots growing along 
the bottom in this debris curled and developed hair in some places, 
but were smooth where they curved up into the water. 
CONTACT. 
Concerning the effect of contact upon the production of root 
hairs authors differ. ScHWARzZ (75, p. 160) offers no explanation 
for their production in the case of water roots of Nuphar or Elodea 
entering the substratum, but thinks they are not due to contact, 
chemical stimulation, or retardation of the growth of the root. On 
air roots of aroids and orchids dry contact produced no hairs, nor 
was he able to cause them on Elodea (presumably in water) by con- 
tact with glass beads or ground glass. In air roots he speaks (p. 120) 
of the suppression of hair by too close contact. Ewart (14, p. 237) 
makes the statement that “for the formation of root hairs on the root 
tendrils (of Vanila) moisture is essential, darkness and contact 
accelerating, and light and dryness retarding it.” PreFrER (64, 
p- 156) denies the effect of contact, attributing the hair development 
to greater moisture near the support. Went (85, p. 8) also thinks 
that not, contact but moisture and absence of light are the factors. 
. In the experiments here reported there was a production of hairs 
on the roots of Elodea not only when the root grew into the mud at 
the bottom of the aquarium, but also when ground quartz was sub- 
stituted for the mud, while in every case roots growing freely in the 
water produced no hairs. Several of these straight, smooth roots 
were allowed to grow into ground quartz, and the tips were found 
usually much bent and curved, and in all cases covered with hairs. 
Sections were made from roots in water, soil, and quartz, and the 
cells were measured. The averages were: 
E AV. LENGTH IN MM. OF CELLS 
MeEpIum Ce Ser ees He ee ees 
ConDITION 
Of epidermis Of cortex 
Wate i 6 8A; 0.104 0.160 Smooth 
APUME Es «oes anes 0.091 0.110 Hairy 
Oise a 0.068 ©. 100 Longer hairs 
