22 BOTANICAL GAZETTE (JULY 
* A th per Av. growth per Av. length of cells | Approximate no. of 
Medium | hg Siglo “- “eg one; in 9 | in mm. cells per day 
ig iss i 8.6 0.36 0.067 128 
Water ices 11.2 0.45 0.085 132 
The lengths of the cells are averages from measurements of air 
and water roots twenty-four hours old, having the same length as 
the average growth per day in the respective medium. We find 
that the rough approximation of the number of cells formed per day 
gives about the same result for the two media; consequently the 
difference in length of the roots and in the rate of growth is due to 
the greater stretching of the cells in the case of the water roots. 
B. Retardation by mechanical means. 
Concerning the effect of retarding growth by mechanical means, 
SCHWARZ (75, p. 159) thinks it is impossible to produce hairs in this 
manner. He was not able to cause them to develop by stopping the 
growth of the root by wire gratings, nor in general by narrow tubes. 
The fact that the wire might have had a toxic effect would discredit 
the former method of experimentation. He does not consider the 
resistance of the earth to be a cause for hair production (p. 160), 
but states that it results in developing hairs nearer the tip. While 
this may not be due to a greater number of cells producing hairs, 
it at least indicates the favoring effect of resistance, in that hairs 
elongate in a region which otherwise only shows the papillae. 
A pot of corn was placed in the top of a glass cylinder, with the — 
roots passing through the bottom and entering the water. One 
root grew horizontally and struck the side of the vessel, becoming 
kinked and hairy. On May g it was drawn away from the glass, 
and on the next day showed a smooth space. On May 12 the root 
again reached the glass, and on May 15 showed hairs. The jar was 
darkened and a glass rod was placed under a smooth vertical root, 
as in diagram, jig. zz. On May 14 the root showed hairs, but had 
swung free and was growing smooth. A plaster cap was unsuccessful, 4g 
as it killed the tip of the root. With the death of the tip many laterals 
grew out producing hairs, some touching the glass and bending, 
and some becoming kinky in free water. 
A second cylinder was set up as in fig. 11. The glass rod was _ 
Sia = IRR San eee 
