INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



265 



is evidence in favor of the conclusion that the first effect of the stimulus is that 

 of a release. By intermittent stimulation (by means of a specially constructed 

 clinostati) the presentation time may be made still shorter. 



The angle assumed by leaves with reference to the stem is influenced by 

 gravitation as well as by Ught. In Fig. 140, B, is shown a Coleus plant that has 

 been rotated on a horizontal shaft parallel with its own axis for twenty-four 

 hours. The leaves are seen to be bent backward toward the stem in a charac- 

 teristic way. In the plant that has stood upright (Fig. 14c, A) the leaves are 

 nearly perpendicular to the stem. 



Fig. 140.-:— Coleus plants, in usual position (A) and after rotating for 24 hours (B), showing 

 , difference m leaf position. {After Pfeffer.) 



Gravitation also frequently controls the position of floral parts, ^ as for in- 

 stance the stamens and pistil of Amaryllis formosissima. When the flower bud 

 opens under usual conditions these organs are directed downward (Fig. 141, 

 at the left), but if the bud is allowed to open in an inverted position (Fig. 141, 

 at the right), the stamens and pistil assume the same position with reference to 

 the earth, but the opposite direction with reference to the reniaining floral parts. 



Plants that normally bear zygomorphic flowers may be made to produce 

 actinomorphic ones if they are rotated in the proper manner during the de- 

 velopment of the flowers. A zygomorphic flower is capable of being divided 



1 Fitting, 1905. [See note 3, p. 262.] 



2 Vochting, Hermann. Ueber Zygomorphie und deren Ursachen. 



Jahrb. wiss. Bot. 17: 297-345. 1886. 



