J. H. Priestley 151 



effects produced by illuminating gas or ethylene upon the etiolated 

 epicotyl. The structural effects described immediately suggested that 

 the primary endodermis was no longer being developed and that con- 

 sequently further growth was accompanied by lateral leakage of the 

 sap into the cortex through the newly differentiated stem tissue near 

 the growing point. The result of the arrival of these solutes in the cortical 

 region of the stem was immediately shown by increase in girth with less 

 development in length, and, as is shown later, the diagetropic curvature 

 might also be anticipated. 



The fact that this toxic action of illuminating gas depended upon 

 the unsaturated hydrocarbons present in it was of immediate significance 

 in other investigations in progress. In conjunction with Miss E. North (ii c) 

 the writer had arrived at the conclusion that the peculiar properties of 

 the Casparian strip could only be explained on the assumption that it 

 was impregnated with fatty substances. Its staining reactions showed 

 that it was usually also lignified and that it retained acid properties. 

 The difficulty with which the fatty substances were removed, suggested 

 that they were condensation or oxidation products of fats or rather, in 

 view of their acid reaction, of fatty acids. Observations with Mr E.. M. 

 Woodman (as yet unpublished) upon the broad bean had shown that 

 the germinating seed contains at the root tip before germination 4 per 

 cent, of fat, and that the fat included both oleic and linoleic acid, though 

 not in sufficient concentration to cause a rapid " drying" of the extracted 

 fat on exposure to air. The peculiar properties of the Casparian strip 

 suggested therefore that the unsaturated linoleic acid present in the root 

 tip, during the tissue differentiation in the region behind the growing tip, 

 diffused from the vascular strand across the cylinder of cells representing 

 the potential endodermis. Here in some way the unsaturated acid was 

 picked out from the saturated acids, and thus concentrated in the walls 

 of a layer of cells abutting upon intercellular spaces containing air, it 

 oxidised and condensed, practically to a varnish, which impregnated the 

 Casparian strip and was responsible for its characteristic behaviour in 

 restricting the diffusion of an aqueous sap. Its sharp localisation at the 

 strip suggested that the diffusing unsaturated acid must be picked up 

 in this region by chemical means, and the acid reaction of the strip 

 indicated that the chemical union was not with the carboxyl group. 

 The significant observations of Neljubow^ and of Crocker and Knight, 

 suggest that in the presence of unsaturated hydrocarbons this Casparian 

 strip may fail to form, and thus the structural change in the epicotyl 

 may be produced. The significance of these facts is unavoidable. The 



