Further Observations on the Transpiration, Stomata, 
Leaf Water-content, and Wilting of Plants. 
BY 
R. C. KNIGHT. 
(From the Department of Plant Physiology and Pathology, Imperial College of Science 
and Technology , London .) 
With three Figures in the Text. 
T HE results of investigations on the transpiration stream and the water- 
supply of plants have directed attention to the phenomenon of wilting 
and to its relation to environmental conditions of soil and atmosphere. The 
essential turgidity of the plant is maintained by virtue of the capacity of the 
tissues to maintain within them a sufficiency of water. The turgor equili- 
brium in the cell system is not a simple one. It is complicated by the fact 
that it is not a static balance, but the water is in a state of continual motion, 
passing from cell to cell with the upward flow of the transpiration stream. 
Thus, unless the movement of water is closely regulated, so that as the 
water leaves a cell it is replaced by the absorption of an equivalent quantity, 
there must result fluctuations in the state of the turgidity of the cell. 
Inability of the cell to obtain sufficient water to counterbalance the loss by 
transpiration or translocation results in wilting. It is evident at the outset 
therefore that the turgidity of the plant is influenced by the factors govern- 
ing water loss and water absorption, viz. the atmospheric evaporating power 
and the quantity and rate of translocation of water through the soil and 
through the plant. Much attention has therefore been focused on the 
questions of wilting coefficients and available soil moisture. Tentative 
efforts have also been made to interpret the wilting process in terms of the 
functions of the plant. Beyond recognition of the fact that wilting involves 
a decrease in water-content, little progress has been made in the analysis of 
the phenomenon. Distinction has been drawn between the early stages of 
the process, ‘incipient wilting’, and the later stages, known simply as 
‘ wilting ’. It is not at all clear how it is possible to determine the line of 
demarcation between these two stages of the process, and there is, moreover, 
no evidence that there is any reason for such demarcation. It may be 
urged that ‘wilting’ implies the visible collapse of the leaf, whilst ‘incipient 
[Annals of Botany, Vol. XXXVI. No. CXLIII. July, 1922.] 
