J. H. Priestley 149 



in Germany is frequently very high, whilst in America (where water gas 

 is the illuminant) it is also usually higher than in this country; this 

 probably provides the reason for the phenomena being first on record 

 in Germany and then found of practical importance in America, whilst 

 they are nearly ignored in this country. 



WheD ethylene is used alone in air the experiments of Knight and 

 Crocker (6) show that a concentration of one part in ten million will 

 retard the growth of the etiolated epicotyl of the pea, whilst a con- 

 centration of four parts in ten million will produce the full triple response, 

 retardation of growth, increase of girth and the diageotropic position. 

 Even w^th the illuminating gas at Leeds, which contains a relatively 

 small concentration of ethylene (about 2 per cent.), it is very easy to 

 obtain the full effect when growing etiolated seedlings in a laboratory 

 where gas is frequently used, and experimentally it can be induced with 

 the utmost ease. It was the consideration of the experiments of Knight 

 and Crocker upon the etiolated pea seedling that led the writer to the 

 experimental work which may go far to explain the mechanism of the 

 effect produced. 



The Mechanism of the Toxic Action of the Gas. 



The significance of the structural effects produced by ethylene upon 

 the etiolated epicotyl of the pea was immediately evident to the writer 

 in the light of some observations recently made on etiolated plants. This 

 work is now being published (ii), but it is necessary to state here some 

 results obtained as to the structure of the etiolated epicotyl of the pea 

 and as to the function of an endodermis. In conjunction with Dr J. 

 E wing (11/) the writer has found that in many plants grown under 

 etiolation conditions, the stem contains a well marked functional primary 

 endodermis from the base of the stem to just behind the growing apex. 

 In the stem of the same plant grown in the light such an endodermis is 

 only present for a very short distance above the ground level. It will 

 be shown elsewhere that in these plants the special structural and mor- 

 phological features characteristic of etiolation, are largely the result of 

 the presence of this endodermis. 



By a primary endodermis is meant an unbroken cylinder of cells in 

 which the Casparian strip forms a continuous network in the substance of 

 both transverse and longitudinal radial walls. An examination of the htera- 

 ture shows that the presence of this primary endodermis in stems grown 

 in the dark has occasionally been noted, but its widespread occurrence 

 under these conditions and the significance of its presence have been 



