152 Toxic Action of Traces of Coal Gas upon Plants 



unsaturated hydrocarbons diffusing into the root, appear to saturate 

 the chemical linkages which usually pick up the unsaturated fatty acids, 

 so that the latter are no longer held up in the region of the future 

 endodermis. 



This argument has been placed first because it preceded in the case 

 of the writer any experimental work with illuminating gas. It only 

 remains to add that experiment and observation support the conclusion 

 thus drawn. Observations were first made upon broad bean seedlings 

 growing in darkness. The epicotyl then develops tall and turgid, round 

 or oval in outline, with a functional primary endodermis reaching to 

 within a centimetre of the growing point. Seedlings were grown over 

 water in bell jars into which a few cubic centimetres of illuminating gas 

 were bubbled. Within three days the epicotyl was evidently swelling in 

 girth as compared with the control. In another day or two the outline 

 of the stem in the "gassed" plant was nearly square in cross section, 

 just as in the normal bean plant grown in daylight. This square stem, 

 the writer had already learnt (ii /), to associate with the disappearance of 

 the endodermis. With the help of Miss L. M. WofEenden a complete 

 structural examination of the seedling was made, when it was immediately 

 clear that the base of the swollen region coincided with the appearance 

 of gaps in the functional endodermis, which lower down in the stem 

 formed an unbroken ring, whilst a little further up in the swollen region 

 every vestige of Casparian strip had disappeared. The disappearance of 

 the Casparian strip under gas poisoning has also been seen in the etiolated 

 epicotyl of the pea. 



It was interesting to note that the gaps in the endodermis appeared 

 first opposite the spaces between the vascular bundles. This was to 

 be expected as the fatty acids used in the formation of the Casparian 

 strip diffuse outwards from the vascular strands towards the endodermal 

 region. Clearly then these acids will first be anticipated in arrival by 

 the ethylene at points in the endodermal ring which are most distant 

 from the vascular strands. 



The observations of Richter(i5) upon the histology of the swollen 

 zone of the pea epicotyl are necessary corollaries of the disappearance 

 of the functional endodermis. With the insurgence of the sap from the 

 vascular strands into the cortical regions collenchyma formation follows, 

 whilst it will be shown elsewhere (ii e) that the formation of active cork 

 meristems in cortical regions requires the free access to the meristem of 

 the organic solutes from the vascular strands. 



The curvature of the etiolated epicotyl or shoot described by 



