150 Toxic Action of Traces of Coal Gas upon Plants 



completely neglected. This may be partly due to the fact that the 

 presence of the strip may easily be missed unless the sections are treated 

 with a special view to its demonstration. Many methods will be found 

 excellent for this purpose; sections cleared by boiling in potash or in 

 Eau de Javelle, washed, and then stained in phloroglucin and hydro- 

 chloric acid, or in gentian violet or safranin, will show the strip admirably. 

 In sections that have not been cleared, the staining reaction of the strip 

 is more difficult to observe, but its presence is often indicated by the 

 occurrei]ce in the endodermal cells of contracted protoplasm which runs 

 always tangentially across the cells remaining in contact with the walls 

 in the region of the strip. In fresh sections stained in phloroglucin and 

 then mounted in concentrated hydrochloric acid, the endodermis is 

 usually very conspicuous, this arrangement of the contracted protoplasm 

 giving it the appearance of a continuous dense thread forming a complete 

 ring. 



The writer has recently spent a considerable amount of time investi- 

 gating the behaviour of the primary endodermis (Priestley (12) and (ii c)). 

 Such a cylinder of tissue may be visualised as a chimney in which the 

 bricks are protoplasts while the Casparian strip represents the mortar 

 between the bricks. This cylinder encloses the vascular strands, within 

 which sap is moving through the plant. Organic solutes are frequently 

 present in the sap, which are of the utmost significance in relation to 

 the growth of the tissue of the plant, continued merismatic growth being 

 impossible unless these solutes are freely supplied. The sap contained 

 within the vascular cylinder will diffuse outwards by way of the walls 

 between the protoplasts and will thus reach the endodermal cylinder. 

 Whether it can reach the tissues outside this depends upon the structure 

 of the endodermis. When an endodermis has the primary structure 

 described above the organic solutes are unable to leak out, because the 

 endodermal protoplasts are relatively impermeable and permit very few 

 solutes, and those mainly inorganic, to pass; whilst the Casparian strip 

 appears to be impregnated with fatty substances and is therefore rela- 

 tively impermeable both to water and to substances dissolved in water. 

 In a stem with such an endodermis then the growth and structural develop- 

 ment of the tissues outside the endodermis is severely restricted. 



The peculiar appearance of etiolated plants with their elongated thin 

 stems and undeveloped leaves may therefore be attributed in part to 

 the presence of a functional endodermis within them, restricting the 

 supply of sap with nourishing solutes to the tissues within the endo- 

 dermis. This idea receives remarkable confirmation from a study of the 



