284 PHYSICAL FACTORS [CH. xxir 



Both common observation, and critical experiments such as 

 these, leave no room for doubt about the fact that accommodation 

 of petiole-length to water-depth does actually occur; but when 

 we pass on to the question of the factors which bring about this 

 accommodation, by causing cessation of growth at the appro- 

 priate moment, we find ourselves on controversial ground. One 

 point seems to be uncontested namely that, in the case of 

 Hydrocharis, the regulation is not due to the change in light 

 intensity, for even in darkness the petioles grow only to exactly 

 the right length to bring the blade to the surface. Frank's 

 experiments led him to the conclusion that, when the lamina 

 reached the water-surface, the lowering of pressure, due to the 

 absence of a superincumbent layer of water, was the physical 

 factor which gave the signal to the petiole to cease growth. 

 However, the repetition and critical analysis of Frank's experi- 

 ments seem to have shown clearly that his deductions cannot 

 be accepted. Karsten 1 , using Ranunculus sceleratus^ Marsilea 

 and HydrochariS) showed that if tubes of oxygen-free air were 

 inverted over individual leaves, the growth of the petiole con- 

 tinued after the lamina had come in contact with the gas, in- 

 stead of ceasing, as it did under normal conditions, as soon 

 as the lamina reached the surface. His experiments seem to 

 justify the conclusion that it is contact with the oxygen of the 

 atmosphere which checks the further growth of the petiole, 

 but we have no conception of the exact nature of the process by 

 which this inhibition is brought about. 



1 Karsten, G. (1888) ; see also Vries, H. de (1873). 



