92 AERATION AND AIR-CONTENT. 



and with further pumping partially unfolded. After 2 hours in the 

 receiver the plants responded again to mechanical stimulus, but 

 with 12 hours in vacuum their irritability had disappeared. The 

 leaves of Mimosa pudica failed to show sleep movements, and the 

 heads of Leontodon taraxacum and Sonchus oleraceus were unable 

 to close or to open in rarefied air, while " sleeping" leaves of Robinia 

 pseudacacia did not open in water free from air. 



Payer (1842 : 1194) found that seedlings of Lepidium sativum 

 exhibited distinct phototropic curvatures in hydrogen or nitrogen, 

 as well as under water, but these were reduced in degree. 



Kabsch (1862 : 341) investigated the effect of carbon dioxid and 

 other gases upon irritability. In pure C0 2 the sensibility of fila- 

 ments of species of Berberis to mechanical stimuli ceased almost 

 instantly. It returned in the air after a few minutes, when immer- 

 sion in the C0 2 had lasted not more than 5 to 10 minutes, but only 

 after several hours when immersion had lasted for 3 to 4 hours. 

 Even after 6 to 12 hours receptivity had not completely disappeared^ 

 A mixture of 30 to 40 per cent of carbon dioxid with air was without 

 effect, but irritability disappeared under greater amounts. The 

 inflorescence of Berberis developed buds and flowers normally after 

 2 to 3 days in pure C0 2 , but a longer stay produced ill effects. Oxalis 

 acetosella and 0. corniculata in carbon dioxid failed to show the nor- 

 mal sleep movements of the leaflets. After being brought into the 

 air, the movements were not resumed until 3 days had elapsed. 

 Movements of the leaflets occurred at a pressure of 15 mm., but at 

 2 to 3 mm. they were no longer irritable. The leaves of Oxalis and 

 the heads of Bellis suspended their periodic movements in vacua, but 

 resumed them upon renewed access to air. 



Wiesner (1878 : 58) subjected seedlings of Lepidium sativum, Pha- 

 seolus multiflorus, Sinapis alba, and Vicia faba to an atmosphere 

 free from oxygen. No curvature occurred after exposure to the 

 light for several hours, but when air was permitted to enter it became 

 visible in an hour. His conclusion was that heliotropism is impos- 

 sible in the absence of oxygen. 



Wortmann (1879:509) placed germinating seeds of Phaseolus multi- 

 florus, P. vulgaris, and Vicia faba in an oxygen-free atmosphere, 

 which completely inhibited geotropic curvature in the horizontal 

 roots, as well as in the stems of Pceonia peregrina. He stated that 

 irritability is entirely lost when oxygen is lacking, but that it returns 

 with renewed access, even after deprivation for weeks. In a special 

 study of geotropism (1884 : 705), seedlings of Helianthus, Lepidium, 

 and Phaseolus were placed in a partial vacuum. The slight curva- 

 ture already begun ceased after a short time, and could not be 

 again induced by the admission of air. Shoots exposed to hydrogen 

 in a horizontal position exhibited no geotropic after-effect when 

 placed in the air, in spite of further growth in length. Detmer 



