Physiological Studies in Plant Anatomy 223 
cases. In the case of the roots, the sap accumulating under pressure 
in the endodermal cylinder contains supplies of organic materials 
from the seed reserves or from the leafy area above ground, and is 
in contact with the closely massed cap of meristematic tissue 
through which it can diffuse but slowly if at all. It follows that 
there will be a tendency for the portion of the meristem abutting 
directly on the endodermal cylinder to be more adequately supplied 
than the tissue nearer the surface of the root. 
On the other hand, at the stem apex it is evident that the supply 
of sap containing the organic solutes necessary to growth, is not so 
restricted. Here the endodermis is absent and the intervening walls 
of the meristem also permit of freer diffusion around the meri¬ 
stematic protoplast. The whole area of the meristem will therefore 
receive a more proportionate supply of the available solutes. 
Nothing is known as to the nature of these solutes, but it is probable 
that they will be amphoteric, nitrogenous compounds in colloidal 
solution, containing the necessary amino (—NH 2 ) and carboxyl 
(—COOH) linkages to allow of their ready employment in syn¬ 
thesis upon adsorption at the surface of the protoplast. It has been 
found (Priestley and North (21)), that the endodermis, when in the 
primary stage and possessing only the Casparian strip, permits the 
ready diffusion of electro-negative colloidal ions whilst it retains 
those with an electro-positive charge. 
. In the roots, so far as our observations go, the sap is normally 
upon the acid side of true neutrality and the hydrolytic products 
of the reserve proteins of the seed usually have an isoelectric point so 
far upon the alkaline side that in such a sap they will behave as 
bases and exist as electro-positive (basic) groups. These groups, 
then, will be unable to escape from the sap of the endodermal 
cylinder and will therefore be carried with the sap down to the 
apical meristem, where they will be as readily absorbed by the 
meristematic protoplast as was the electro-positive colloidal colour 
group of the basic dyes (see p. 220). On the other hand, in the stem, 
the sap, at any rate in the neighbourhood of the growing point, is 
apparently more alkaline in reaction, so that the same products of 
protein hydrolysis will now behave as electro-negative or acid 
groups (Loeb (io,ii)) and as we have seen the meristematic cells of 
the stem apex are distinguished by their ready staining with acid 
dyes. These electro-negative groups will be able to escape through 
the tardily formed endodermis, or its substitute the starch sheath, 
and here we may have the explanation of the much greater exo- 
