1902] THE RHEOTROPISM OF ROOTS 273 
A comparison of the optimum temperature for growth with 
the temperature used in the experiments will indicate that the 
seedlings used would probably, in most cases, show in the opti- 
mum temperature a shorter latent period than that given.s We 
may suppose that by the aid of the reading microscope the 
latent period could be seen at the optimum temperature to end 
for sativus, Brassica alba, and Hordeum vulgare in an 
hour. It is hardly to be supposed, however, that for Pisum 
satioum, Lathyrus odoratus, and Vicia faba, the latent period could 
be reduced to two hours. Compared with the latent period in 
Seotropic response, the periods for rheotropism are extremely 
long. The roots of Raphanus sativus when laid horizontally in 
wate, ata temperature of 26°, show to the unaided eye a geo- 
‘topic bend in 15 minutes ; those of Pisum sativum, similarly 
‘reated, show, at a temperature of 23°,a geotropic bend in 20 
Minutes. The latent period for rheotropism, therefore, is six 
HMes as long as that for geotropism. A rheotropic curve, how- 
ne Carried out in Opposition to the geotropic tendency of . 
hed it. No one has yet found the latent rheotropic period 
When Stavitation is neutralized. A better comparison can be 
a with, the heliotropic latent period, which is shown by the : 
Pant while stil under the influence of gravitation. : oo 
Oo ems of Brassica alba and Helianthus annuus have their 
< ‘mmersed in water in an air temperature of 23°, and are 
Po esa to one-sided illumination with strong, diffused — ; 
POR gia latent Period will be found comparable to that in — 
2 POW seedlings of Brassica alba were used, and the © 
frst negative bend it in: 75. 
i. 5. perceptible to the naked eye page eas ae : 
oe Within two hours of the beginning, all four roots had ae 
ee. The heliotropic curves in Helianthus 
ore hours had elapsed; then only three 
' of five had responded. <6 ee 
Song Wiss. Bot. 3a: 195) shows that the optimum temperature for 
nie Gsckest geotopic response ee 
