t 41 ] 
FURTHER OBSERVATIONS UPON THE 
MECHANISM OF ROOT PRESSURE 
By J. H. PRIESTLEY 
I ' n a recent number of this journal, V. H. Blackman(2) makes fre¬ 
quent critical reference to a recent attempt made by the present 
writer (li) to explain the mechanism of root pressure in terms con¬ 
sistent with the facts and with physical and chemical laws. Since 
the publication of the paper referred to the writer has continued the 
experimental study of exudation pressures and some results of these 
studies now await publication. In the meantime a brief re-statement 
of the main principles involved in the original hypothesis seems 
desirable, in view both of Blackman’s criticisms and of other work 
to be mentioned later. 
Reference to the earlier paper will show that the mechanism 
suggested requires the fulfilment of the following structural and 
physiological conditions if an adequate exudation pressure is to be 
maintained in the xylem vessels of the root. 
1. A continuous chain of protoplasts with living semi-permeable 
membranes, permitting of the movement of water inwards to the proto¬ 
plasts adjoining the xylem vessel by the ordinary process of osmosis. 
2. A continuous supply of solutes to the xylem vessel to replace 
those carried upwards by the ascending column of sap. 
3. An endodermis in which the network of Casparian strip per¬ 
forms two functions, viz. (a) It forms a rigid framework preventing 
too much extension of the tightly packed protoplasts within the 
endodermal cylinder, (b) It prevents leakage of the sap, and therefore 
of the solutes, from the xylem through the walls intervening between 
the protoplasts. As the protoplasts of the endodermis are themselves 
relatively impermeable, this structural peculiarity of the endodermis 
renders practically negligible the loss of solutes from the xylem into 
the cortex. 
On all these features of the mechanism it is desired to add a few 
comments. 
1. Osmosis and Water Absorption 
Blackman(2), loc. cit. p. hi, is certainly justified in drawing 
attention to the neglect of Thoday’s useful paper ( 13 ). The phraseology 
adopted in that paper makes it admirably clear that the passage of 
