192 TEANSACTIONS LIVEEPOOL BIOLOGICAL SOCIETY. 



penetrating the upper plug of cotton wool, then bends 

 downwards again and this may be repeated more than 

 once. This upward curvature, followed by a downward 

 one, is due to the fact that the radicle at first bends 

 upwards in virtue of its oxytropic irritability until having 

 penetrated the upper plug of cotton wool, it reaches and 

 is surrounded on all sides by an atmosphere rich in 

 oxygen, when in virtue of its geotropic irritability it 

 curves downwards again. In a few cases the radicles 

 grow downwards into the lower cotton wool for a short 

 distance, growth becoming slower and slower as the 

 radicle penetrates deeper and deeper. In such cases the 

 tip of the radicle was probably equally surrounded on all 

 sides by an atmosphere poor in oxygen and rich in 

 C0 2 and hence being equally affected on all sides its 

 oxytropic irritability or tendency to bend towards regions 

 richer in oxygen was not called into play. Experiments 

 performed with a variety of seeds gave in general corro- 

 boratory results and though the oxytropic irritability is 

 not always equally well marked, nevertheless it can 

 generally be made under appropriate conditions to over- 

 come both geotropism and somatropism. 



In Helianthus annuus the primary root shows marked 

 oxytropism, which in the secondary roots is only slight 

 and is weaker than their somatropism. Pisum is similar 

 to Helianthus but the oxytropism of the secondary roots 

 is rather more pronounced. In Gucurbita the radicles 

 show distinct oxytropism and but slight somatropism but 

 in the secondary roots the somatropism is marked and 

 the oxytropism only slight. The primary radicles of 

 Hemp also show oxytropism but their somatropism is 

 very strong and in the secondary roots quite overpowers 

 the other directive agencies. 



That roots develop and flourish more abundantly in 



