362 Knight . — Further Observations on the Transpiration , 
wilting 5 consists merely of a decrease in leaf water-content without visible 
collapse. Such a criterion, susceptible as it is to personal error, can hardly 
be admitted as a help to a real analysis of the problem. Consequently, in 
the present paper, the term ‘ wilting 5 simply denotes a decrease in water- 
content. 
The concept of ‘ permanent wilting ’ h; c been employed by Shive and 
Livingston in a study of the wilting phenomena. A plant is said to be per- 
manently wilted when transference to an atmosphere saturated with water 
vapour fails to revive it within twenty-four hours. The difficulty and labour 
of the determination of the permanent wilting-point are self-evident, and 
have militated against the use of this method. Bakke (1, 2) has preferred 
to adopt changes in the transpiration rate as criteria of the progress of 
wilting. He used Stahl’s cobalt chloride method, as modified by Shreve 
and Livingston, for determining transpiration rate, and found that as the 
plant wilts this rate falls steadily to a minimum, which is maintained for 
a longer or shorter period, depending upon the species. Following this 
period the transpiration* rate rises rapidly to a maximum before the final 
decrease accompanying the drying out of the plant. Bakke attributes this 
sudden increase in transpiration rate to the sudden rupture of the water 
columns in the plant, and the point at which this occurs is considered to be 
the permanent wilting-point. No evidence has been adduced, however, to 
show that this point coincides with, or indeed has any relation to the 
permanent wilting-point as defined by Shive and Livingston. It appears 
that experimental demonstration of the sudden rupture of the water columns 
in the plant, postulated by Bakke, would prove difficult. The permanent 
wilting-point of Shive and Livingston may be defined as the stage during 
the wilting process at which the process of diminution of cell water-content, 
hitherto reversible, becomes irreversible. It is of course conceivable that 
the passage of the wilting process from the reversible to . the irreversible 
stage may be accompanied by some sudden internal change such as that 
postulated by Bakke, but at present there appears to be no evidence of the 
occurrence of a sudden and complete rupture of the water columns such as 
might be responsible for a sudden increase in the rate of transpiration. 
The presence of air gaps in the water columns at certain seasons of the year 
is undoubted. Dixon (10) has dealt with this point at some length, and 
Farmer (11, pp. 245-7) has traced the gradual disappearance of air from the 
wood in autumn as indicated by the increase of density. This increase was 
shown to spread gradually from the basal shoots to those situated nearer 
the apex of the tree. The bubbles of air appear again in the wood during 
the summer as the result of the tension set up by the evaporation of water 
from the leaves, but at present there is no experimental evidence known to 
the present writer to show whether the apt ^arance of bubbles is a sudden 
or a gradual process. A priori , one would xpect that the quantity of air 
