J. H. Priestley 153 



Neljubow{9, 10) and Singer (16) as a diageotropic response and by 

 Molisch{8) and Richter(i5) as the combined result of decreased geotropic, 

 and increased heliotropic sensibility, require further examination. The 

 etiolated shoot has always a sharply curved apex which only becomes 

 erect upon exposure to light. The sharp curvature must be an indication 

 of different rates of growth in the two sides of the apical meristem, the 

 inner side of the hook at the apex presumably finding greater difficulty 

 in development. It is only natural, therefore, that differentiation behind the 

 growing apex should also proceed at unequal rates on different sides and 

 that as a consequence the upper rim of the endodermal cylinder should 

 not be at the same level all round the stem. Usually it may be expected 

 to lag behind in development on the side which develops from the inside 

 of the hook. Naturally the failure of the endodermis in the gassed plants 

 usually occurs at a lower level on this side, and extension of cortical 

 tissues follows therefore most rapidly on this side, and the epicotyl is 

 bent over with the hooked apex on the upper side of the horizontal 

 portion. While this is the usual curvature of the horizontal stem it 

 is by no means invariable, the position of the first break in the endo- 

 dermis is also very irregular and probably determined by a varying 

 combination of internal and external factors. 



The explanation just presented of the mechanism of gas poisoning 

 accounts adequately for the sensitiveness of the etiolated epicotyl and 

 the insensitiveness of the normal stem in the same plant, as in the normal 

 stem no functional endodermis is present. It also accounts adequately 

 for the poisonous effect of traces of the gas upon roots of the higher 

 plants where a primary endodermis is invariably present in the growing 

 region. Here again, examination in the case of the broad bean has shown 

 the primary endodermis broken through as a result of the effect of the 

 gas, and the consequent tubercular swelling is clearly due to the abnormal 

 supply of nutrient sap to the cortical tissues just behind the growing 

 point. The importance of the primary endodermis in the development 

 of sap pressures by the root has been emphasised elsewhere (12), and 

 reference to these papers will show that in the opinion of the writer this 

 action of illuminating gas would ultimately prove fatal to the plant, 

 as the supply of sap to the growing aerial portion of the plant depends 

 very largely upon the exudation pressures in the root. 



At present the writer is not in a position to discuss the interesting 

 effects of traces of coal gas upon the flower bud of carnation, or the 

 epinastic movement of the leaf of Ricinus, but the effect produced 

 upon cork (p. 148) appears to be closely related to the effect produced 



