36 
J. H. Priestley and J. Ewing 
especially in the potato, the endodermal cylinder is very strongly 
distended by the exudation pressures generated by the sap within. 
If the stems are cut off and glass tubes attached by rubber con¬ 
nections, relatively large quantities of sap can soon be collected. 
Under these conditions any weak places in the endodermal cylinder 
will obviously be severely tested and will possibly break down. It 
is suggested that the endodermal cells, just above the insertion of 
the leaf rudiment, represent such weak spots, that frequently the endo- 
dermis breaks down in this region under the strong internal hydro¬ 
static pressure, and that the cortical region is then irrigated with 
nutrient sap, and a lateral shoot develops. In the case of the bean 
it is suggestive that the lateral branches usually appear at upper 
nodes where, as has already been pointed out, the endodermal cylinder 
fails to form. 
It is also inevitable that the sap pressure in etiolated shoots should 
have a marked influence on growth. In comparison with the normal 
shoot our experiments make one point quite clear: the reduced nature 
of the transpiring organs of etiolated shoots, and the depression of 
transpiration incidental on darkness cause a more or less constant 
sap pressure, so that growth in length or extension goes on equally 
by day and by night. In the green shoot, however, extension goes on 
for the most part by night, and is very much reduced or comes to a 
complete standstill during the day. In the case of Phaseolus multi- 
florus, on which some of our measurements were made, for the first 
two weeks of growth the nightly increments in extension in the green 
seedlings were equal to the daily or the nightly incremehts in the case 
of the etiolated seedlings. Thus in the early stages the rate of exten¬ 
sion in the etiolated seedlings was approximately twice that of the 
green seedlings. As growth proceeds, however, the development of 
side axes and leaves on the green seedlings renders comparison of the 
relative rates of growth practically impossible. 
of etiolation, and to consider separately the problem as it aliects leaf 
and stem respectively. Sachs also distinguished between the different 
types of reaction to continued darkness shown by plants of various 
