196 
]. H. Priestley. 
a dye, if it cannot penetrate the protoplasm, does not penetrate 
through the walls further inwards than the endodermis have been 
confirmed. In the course of these experiments it was noted that 
at the root tip the penetration inwards via the walls was even 
less marked. To obtain the result it is necessary to experiment 
with roots that have been previously grown in water: otherwise, 
when the roots are transferred to the dye solution toxic conditions 
are invariably produced and the dye penetrates right through the 
tip, freely entering the dead cells. 
These observations have been made on willow roots and onion 
roots, using various dyes chosen for their penetrability on the basis 
of the valuable data given by Ruhland (9, 1912). Good results 
have been obtained with light green PS, orange G, neutral red and 
malachite green. 
These dyes penetrate cellulose walls readily and in some cases 
can penetrate the living protoplasm. Their inability to penetrate 
at the root tip suggests, therefore, that the cell walls in the 
neighbourhood of the apex have the properties of protoplasm 
rather than of dead cellulose walls, at any rate with regard to 
diffusion. By the use of such dyes it is also possible to obtain 
ready confirmation of the greater permeability of the apical cells of 
the hydathodes of Phaseolus vulgaris. If a leaf of this plant be 
dipped in dilute (0*05%) aqueous neutral red, for a few hours, and 
then a section of the leaf is examined in water, the plasma of the 
apical cells of the hydathode will be found to be deeply stained, but 
the lower cells of the stalk will not be stained even after 24 hours’ 
immersion. 
That some of the cells within the endodermis likewise have a 
greater permeability relatively to the endodermal cells, has been 
illustrated by similar experiments, so that these cells correspond in 
this respect to the apical cells of the hydathode and the endodermal 
cells to the basal cells. In these experiments young plants are 
taken and their root tips cut across under water and the roots then 
placed in aqueous solutions of various dyes (usually 0*1%). Best 
results have been obtained with acid green, malachite green, neutral 
red, and light green PS. But various seedlings may be used and 
probably the exact combination of seedling and dye for best results 
varies with different external conditions. The solution of the dye 
is then drawn up the xylem and the water previously inside 
the vessels is drawn up to the leaves to replace that lost by 
transpiration. 
