442 PHYSIOLOGY OF GROWTH. 



flower buds were placed horizontally in the moist atmosphere of 

 the box, in darkness, I found that, at a temperature of 24 C., 

 they had even after a few hours curved strongly upwards. The 

 geotropic growth movement finally ceased when the upper part 

 of the stem made an angle of 90 with the lower. Geotropic curva- 

 tures are also easily determined in this way in severed epicotyls of 

 bean seedlings grown in darkness (see Fig. 143). In experiments 

 with Aristolochia Sipho, I used not entire shoots but pieces cut 

 out of actively growing interriodes. They exhibited vigorous 

 negative geotropism, and this fact, which can also be determined 

 without any difficulty in stems of other plants (e.g. portions of 

 the epicotyl of bean seedlings grown in darkness), is of interest 

 in connection with what is said in 175, where the significance of 

 the root tip in the initiation of root curvatures will be under 

 consideration, since it teaches very clearly that the tip of the stem 

 in Phaseolus does not seem to determine exclusively the geotropie 

 behaviour of the stem. The epicotyl of Phaseolus is specially 

 favourable for researches on geotropism, since bean seedlings can 

 be grown without trouble at any time of the year. The power, 

 however, of reacting to the action of gravity is by no means 

 quantitatively the same in different plant structures. While the 



FIG. 143. Epicotyl of Phaseolus multiflorus, exhibiting a negative ^eotropic curvature. 



structures above mentioned curve very quickly and vigorously 

 when they have been placed in a horizontal position in a moist 

 place, young defoliated shoots of Sambucus nigra, e.g., under similar 

 conditions only slowly execute geotropic curvatures. The plumule 

 of Triticum vulgare possesses great geotropic irritability. If small 

 pots in which wheat seedlings with plumules about 2 cm. long 

 have developed, are laid horizontally, the plumules rapidly curve 

 upwards. 



