30 AERATION AND AIR-CONTENT. 



aerotropic rootlets with the same great air-spaces. Perseke (1877 : 27) 

 has shown that a similar development may occur in terrestrial 

 plants, such as Phaseolus muUiflorus. When grown in water, the 

 latter develops great air-spaces in the cortical parenchyma, and in 

 roots 3 months old appear tears, due to the expansion of the air 

 in this tissue. The aerenchym cells are in but slight contact, as 

 in diaphragms, but the spaces are much larger. The aerenchym 

 breaks the epidermis and cortex and its cells are then in direct 

 contact with the medium, but the air is so firmly embedded in 

 the tissue that water can not enter through the rifts. 



The air of the aerotropic roots of Jussicea grandiflora was found 

 by Martins (1866) to contain 87.5 N and 12.5 oxygen, and that of 

 J. repens to have 86.3 N and 13.7 oxygen. Immendorff determined 

 for Schenck that the aerenchym of Lythrum salicaria contained 30 

 per cent of oxygen, and also a small quantity of CO2. In hydro- 

 phytic shrubs without aerenchym, access of oxygen is taken care of 

 by means of numerous lenticels, which produce in many cases a 

 mass of stopper-cells closely resembling true aerenchym. In Salix 

 dminalis the number of lenticels is much greater on submerged 

 than on aerial shoots. The tissue contains air-spaces, is white in- 

 stead of brown as in the air, and makes a plate often 2 mm. high. 

 Similar lenticel plates are found in Eupatorium cannabinum, Bidens 

 tripartitus, Malachra gaudichaudiana, Scoparia dulcis, Aeschynomene 

 sensitiva and hispida, and Solarium sp. When Artemisia vulgaris 

 grows in w^ater, the few cortical layers of the roots develop into a 

 tissue 2 to 3 mm. thick, and very like aerenchym, differing only in 

 not arising from phellogen. Scott and Wager (1888) concluded that 

 the floating tissue of the roots of Sesbania also facilitates the access 

 of oxygen. 



Wilson (1889) observed that the number and size of the "knees" 

 of Taxodium distichum were determined by the height of the water 

 and the duration of flooding. Young roots often turn directly 

 upward until they reach the surface, when they again bend beneath 

 the water. On old trees the roots often grow together and the knees 

 arise at the point of union. In dry soil cypress trees show no trace 

 of such development and there seems no question that they are to 

 be regarded as aerating organs. Similar structures are found about 

 the base of trunks of Nyssa aquatica in swamps, the roots bending 

 sharply upward to a distance of 6 to 8 inches above the surface of 

 the water and then bending downward into it again. 



Ewart (1894 : 238) found that, when peas were suspended above 

 water free from oxygen with the radicles touching, the radicles that 

 pointed downward soon bent laterally or curved sharply upwards, 

 while those pointed upward grew in that direction for a longer period 

 than usual before responding to geotropism. When they reached 

 the water they bent upward or grew along over the surface. Over 



