1918] BAKKE—WILTING tit 
ception of continuous water columns is in force. When the force 
of evaporation becomes sufficient to cause a serious rupture of 
these water columns, then the plant wilts. Just to what extent a 
serious rupture can be regarded cannot be stated, but it must be 
greater than the force of cohesion which holds the water particles 
together. 
The extent of this cohesion force has been sufficiently presented 
and advanced by Drxon (16, 17, 18), RENNER (34, 35, 36), Ur- 
SPRUNG (43, 44, 45, 46, 47), and others (21, 32), and although the 
conclusions have been criticized by Jost (22), nevertheless they 
are substantiated. It is not the province of this article to enter 
into a critical discussion of these various papers. The approxi- 
mate point of permanent wilting is readily ascertained from the 
beginning by taking a series of readings of the foliar transpiring 
power of the plant in question. Care should be taken to obtain in 
the series the maximum and minimum. Although there is not any 
hard and fast relation between the maximum and the minimum, 
when the moisture in a soil has been reduced to the point where the 
maximum is below the normal minimum, at a time of the normal 
maximum, then the water content of that soil has attained what 
the writer designates as the critical content. From this point 
it is simply a question of time when the columns break. This then 
becomes a relatively simple matter. 
The readings giving the indices of foliar transpiring power 
taken at hourly intervals present a graph that is similar to graphs 
set forth previously. The maximum occurs at a time previous to 
the highest evaporation; the minimum generally occurs somewhere 
between the 18th hour and the 24th hour. There is a decided 
drop in the afternoon, which occurs at a time of day when evapora- 
tion is at its height or nearly so. There is a recovery that is also 
conspicuous. The cause for this resistance has been advanced by 
SHREVE (40) as being due to the imbibitional forces of the cell wall 
and of the colloids of the protoplasm. Although this feature has 
been noticed wherever the march of foliar transpiring power has 
been obtained, no one as yet has set forth any evidence as to the 
length of time necessary for recovery to take place. It is appar- 
ent that the recovery has been ‘complete before the time of the 
