Physiological Studies in Plant Anatomy 211 
siderable evidence that this meristematic cap is relatively imper¬ 
meable to the passage of both water and solutes. 
In the next section it is shown that a consideration of the con¬ 
ditions necessary for protoplasmic synthesis in meristematic tissue 
is a priori evidence for the conclusion thus reached experimentally. 
These considerations have led to a comparison of the relative 
permeabilities of the apex of stem and root and this in its turn has 
involved a preliminary study of the development of the cell wall 
in the meristematic region. Conclusions as to chemical changes in 
the cell membrane can only be very tentative in the present state 
of our knowledge but it can be shown that the experimental evi¬ 
dence provides an interesting commentary on the differences between 
the two apices. 
The conclusion that the growing point of the root is relatively 
impermeable is in fundamental disagreement with the statements 
of a recent investigator as to the function of the root apex. A third 
section is therefore devoted to a critical examination of the experi¬ 
mental evidence brought forward by Coupin to show that the root 
apex functions as an absorptive organ, and further experimental 
evidence presented from our own work which supports our alterna¬ 
tive and very different explanation of Coupin’s results. 
I. An Experimental Investigation of the Permeability 
of the Root Apex 
The meristematic cap of tissue closing the endodermal cylinder 
at the root apex is relatively small in extent and not very accessible 
to experimental treatment. Evidence has been obtained along three 
different experimental lines which agrees in suggesting that these 
meristematic tissues are relatively impermeable either to water or 
to solutes. 
(1) Experiments with water under pressure 
Similar experiments have already been carried out by de Vries ( 28 ) 
who was concerned in demonstrating the relative impermeability 
of the endodermis to water. In using this method to investigate 
the resistance of the meristematic cap of tissue to water it was 
desirable to use as short a length of root as possible so that the full 
experimental pressure might be exerted upon the meristem itself. 
It is not possible to make watertight connections between india- 
rubber pressure tubing and the delicate tissue of a growing root. 
The young roots of germinating broad beans (Vicia Faba L.) were 
