July, 1921] OVERTON — MECHANISM OF ROOT PRESSURE 
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carries the solutes along with it. These assumptions are held to contradict 
theory and observation on osmotic movement. The general conception 
of the function of the xylem is that, in times when water is abundant, it 
carries the inorganic substances to the leaves; but Atkins finds that sugars 
are at all times present in the sap of the vessels, usually in greater quantities 
than the electrolytes. Therefore, the vessels should be regarded as trans- 
ferring both water and solutes, organic as well as inorganic. This view, 
however, is opposed to that of Curtis, who suggests that the interposition of 
living cells across conducting tubes may prevent a flow of solution, and that 
water may normally flow largely by diffusion. 
It has, however, been shown that with the possible exception of Colccasia 
secretions contain both electrolytes and non-electrolytes. 
An attempt has been made in this discussion to explain the entrance 
of water and solutes into the vessels in the presence of root pressure. From 
a physical point of view it is difficult to conceive why solutes should not be 
carried with the transpiration current once they enter the vessels. If, 
in the absence of root pressure, and when there exists a negative pressure 
in the vessels, water enters the root by filtration, it is also conceivable that 
soil solutes may also enter along with the water unless some mechanism 
exists in the root to check their entrance. Curtis, however, holds that there 
is no flow of solution from the soil into the root, and that the amount of 
water absorbed bears no relation to the amount of solutes absorbed. Pfeffer 
states that 
A substance imbibed by the cellulose ceil walls may reacii the center of a tissue with- 
out having penetrated a single protoplast. It is indeed possible that the water and salts 
absorbed by the roots pass mainly, if not entirely, through either the cell walls of living 
cells or the walls and cavities of dead wood fibers, etc., so that only on reaching the crown 
of the tree do they penetrate the protoplasts of the actively growing tissues localized there. 
If we admit that soil solutes enter the vessels either by filtration or by 
excretion or both, and are carried to the leaves by the transpiration current, 
some provision must be conceived to prevent their too great concentration 
in the leaf cells, otherwise in the course of a transpiring season the leaves 
would become rigid with salt. Possibly the endodermis may function in 
controlling the entrance of soil solutes as well as in preventing the backward 
leakage of water and solutes from the root. Some evidence already exists 
to support this view, as substances which cannot penetrate the endodermis 
do not penetrate into the vascular cylinder of the root. In fact, De Rufz 
de Lavison shows that the suberized walls of endodermal cells are im- 
permeable, that all solutes which enter the vascular cylinder of the root are 
forced to pass through the protoplasm of the endodermis, and also that 
cellulose cell walls of the root tip are of such a character that only such 
solutes enter this region as are able to penetrate the protoplasm. The 
supposition is, therefore, that the quantity and quality of solutes which 
penetrate into the root depend upon a sort of filtration across the protoplasm 
