6 
Proceedings of the Royal Society of Edinburgh. [Sess. 
young and primitive plants the mantle is unbroken by intercellular spaces, 
even the respiration of the living cells within the barrier can only be 
conducted by interchange of gases passed in solution through the cells of 
the endodermis. These structural facts, which can be verified by sections 
of the stem of any young fern-plant, or of any root, form the foundation 
of a theory which may account for some of the most extraordinary vascular 
developments seen in plants. 
Evidence of the effectiveness of the endodermis as a physiological 
Fig. 4. — Part of transverse section of stem of Acrostichum ctureum , showing the 
centre of the solenostele, with endodermis surrounding a small central pith 
with large starch-grains ; outside the endodermis is conjunctive parenchyma 
with small starch-grakis. Drawn by Dr J. M. Thompson. ( x 66.) 
barrier is afforded by comparison of the cell-contents outside and inside 
it ; sometimes starch is absent from the stelar tissues but present in thfe 
surrounding cortex. Marked cases may commonly be found of difference 
in size of the starch-grains on either side of the barrier. This is seen 
in the storage-rhizomes of Pteridium, and fig. 3 shows it in the case 
of the rhizome of Helminthostachys. In the young stem of Acrostichum 
aureum the difference is still more striking (fig. 4). Such facts indicate 
that the endodermis controls the passage of soluble sugar. It has been 
shown by de Lavison (Rev. gen. de Boh, 1910, p. 225), and by Priestley 
(New Phyt., 1920, p. 192), that it is an effective barrier to the passage of 
such soluble substances as are incapable of penetrating the protoplasm, but 
whose passage through the walls can be followed by their colour, or by 
