RESPIRATION AND OXYGEN. 29 



Aerotropism is regarded as a paratonic nutation, in which the 

 external factors influence the growing region directly, and not indi- 

 rectly through the root-tip, as in hydrotropism. If young seedlings 

 of corn are fastened so that their root-tips touch a water-surface, 

 they show irregular bending in the water, or they turn and grow 

 along the surface. The irregular nutations are due to abnormal 

 influences, among them the lack of oxygen. This is confirmed by 

 the fact that corn roots show exactly the same bendings in air lacking 

 in oxygen or mixed with illuminating gas. The horizontal growth of 

 the roots upon the water is an aerotropic movement determined by 

 the high oxygen-content of the uppermost water-layer. 



Goebel (1886 : 249) pointed out that the two tropical genera, 

 Sonneratia and Avicennia, growing in swamps but not related sys- 

 tematically to each other, exhibit roots which grow upright. These 

 roots were regarded as air-roots, which permitted the roots creeping 

 in oxygen-free mud to come in contact with the air, and therefore 

 as organs of respiration. He endeavored to test this conception 

 experimentally, and found that upright roots developed when Rumex, 

 Nymphaea, etc., were planted too deeply, and that similar results 

 occurred when Saccharum was kept in a very wet pot. These obser- 

 vations confirmed the view that such roots are the result of growth 

 processes operating under a lack of oxygen. 



Jost (1887 : 601) studied the roots of a number of palms and 

 Pandanacece, as well as those of Saccharum, Cypcrus, and Luff a, and 

 reached the conclusion that aerotropic roots serving as organs of 

 respiration have a much wider distribution than has been supposed. 

 This is shown, moreover, in greenhouses, where the roots of Cyperus, 

 Papyrus, Richardia, and Musa appear on the upper surface of pots. 

 It was assumed that the lack of oxygen is the stimulus which pro- 

 duces aerotropic roots. This is shown also by cases where roots 

 regularly grow at the top of the pot and where they form a layer 

 on the inner surface of the latter. Fraxinus and Alnus produce a 

 great number of adventitious roots which run horizontally above 

 the oxygen-free swamp soil, and such air-roots may permit trees to 

 grow in the soil of moors. Aerotropism was cdnsidered to be widely 

 distributed and probably to possess great biological significance. 



Schenck (1889 : 526) gave the name aerenchym to a tissue devel- 

 oped from the phellogen of the shoots and older roots, chiefly of low 

 shrubs growing in swamps or wet soil. It consists of thin-walled, 

 non-suberized cells, with large communicating air-spaces much larger 

 than the cylindric cells. It occurs in a large number of genera, 

 Jussicea, Epilobium, Ly thrum, Cuphea, Hypericum, Cleome, Sesbania, 

 etc. Most of these are shrubs, but several, Jussicea repens, J . natans, 

 Epilobium hirsutum, Ly thrum salicaria, etc., are herbs. In a num- 

 ber also, Jussicea peruviana, J. pilosa, J. repens, J. natans, are found 



