ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 873 



into a glass filled witli water, and its surface kept tiniforinly moist. 

 If the funnel is placed in an atmosphere saturated with moisture, then 

 the roots which project through the perforations in the cover grow 

 vertically downwards ; but when placed in an atmosphere only 

 moderately moist, the roots are diverted from their normal direction, 

 and become closely attached to the conical surface of the moist 

 funnel. 



Influence of Diminislied Atmospheric Pressure on the Growth 

 of Plants.* — Experiments conducted by A. Wieler at Tubingen show 

 that, all other external conditions being the same, plants will grow 

 more rapidly under diminished atmospheric pressure. Thus, if a 

 specimen of the common Windsor bean [Vicia Faha) be grown in a 

 receptacle in which the pressure of the air can be controlled, it will be 

 found to grow faster until the pressure has been diminished to 100- 

 300 mm. ; the normal pressure under which the ancestors of the plant 

 have flourished being, of course, not far from 760 mm. If, however, 

 the pressure is reduced below the smaller figure given above, the rate 

 of growth diminishes. Wieler found that the curve of growth of the 

 sunflower is about the same as that of the bean. It was further 

 shown by his experiments that growth is retarded by increased pres- 

 sure until the minimum is reached at 2-2^ atmospheres, from which 

 point there is again an increase. 



The quantity of oxygen in the atmosphere may in fact be reduced 

 to a very small amount without entirely destroying the power of sup- 

 porting vegetable life. The density at which the maximum growth 

 took place varied with different plants. This may obviously be of 

 advantage to plants growing at great altitudes. 



Eflfect of Radiant Heat on the Growth of Plants.f — Following 

 out the observations of Van Tieghem on the phenomenon termed by 

 him " thermotropism," J. Wortmann has experimented on the influence 

 on the growth of plants of changes in temperature independent of 

 light. He found that rays of heat act on growing parts of plants in 

 very much the same way as rays of light. Eays of heat of a definite 

 intensity falling on one side only cause curvatures in the growing 

 parts of plants which turn either towards or away from the source of 

 heat; and these are quite independent of the effects of I'ght. The 

 apparatus employed was a large chamber in which the source of heat 

 was a blackened iron plate heated by a gas-burner, heliotropic curva- 

 tures being prevented by special contrivances. The plants experi- 

 mented on were Zea Mays, Linum usitatissimum, Lepidium sativum, 

 and fructifications of Phycomyces nitens. Of these, the first showed 

 itself to be positively, all the others negatively thermotropic. In 

 order for thermotropic curvatures to take place, the temperature must 

 be at least 20° C, and the rapidity of the curvature is proportional to 

 the intensity of the rays of heat. 



* Unters. Bot. Inst. Tubingen, i. See Bot. Ztg., xli. (1883) p. 452. Science, 

 ii. (1883) p. 178. 



t Bot. Ztg., xli. (1883) pp. 457-70, 473-80. 

 Ser. 2.— Vol. III. 3 N 



