GROWTH. 367 



The form of experiment adopted by de Vries for demonstrating 

 hyponasty and epinasty was as follows. Petioles and midribs of well- 

 developed but still growing leaves, freed from their laminae, and bilateral 

 shoots which had been previously growing more or less horizontally, 

 were fixed vertically with their basal ends in wet sand and were kept 

 in the dark. The position with relation to the vertical taken up by the 

 apex after some time afforded an indication of the mode of growth of the 

 organ. The most common result was that the apices had deviated from 

 the vertical towards the lower surface of the organ in consequence of this 

 surface having become concave ; in some cases the opposite was ob- 

 served, that is, the apices curved towards the normally upper surface. 

 By this means he ascertained that nearly all leaves and parts of leaves 

 (in this stage of development), the lateral branches of inflorescences, and 

 many horizontal shoots, were epinastic, and that some few midribs and 

 many horizontal shoots were hyponastic. 



Other spontaneous irregularities of growth allied to epi- 

 nasty and hyponasty are exhibited by organs which are not 

 bilaterally but radially symmetrical, and which subsequently 

 exhibit circumnutation. In seedlings in which the cotyledons 

 are epigean, the hypocotyledonary portion of the stem 

 (hypocotyl), as described by Darwin, is strongly arched when 

 it escapes from the seed-coats. This curvature is due to the 

 more rapid growth of one longitudinal half of the hypocotyl 

 as compared with that of the other. The hypocotyl gradually 

 becomes straight in consequence of the increasing rapidity 

 of growth of the side which was originally concave. We 

 cannot properly use the terms "hyponasty" and "epinasty" 

 in these cases, for the hypocotyl is not bilateral. But we 

 may conventionally regard the surface which is at first con- 

 cave as the anterior or ventral surface, and term its period 

 of rapid growth epinasty: similarly, we may regard the 

 surface which is at first convex as the posterior or dorsal 

 surface, and term its period of rapid growth hyponasty. 

 On this convention we may say that the original curvature 

 of the hypocotyl is due to hyponasty, and its subsequent 

 straightening to epinasty. 



Similar phenomena are exhibited by the epicotyledonary 

 portion of the stem (epicotyl) of seedlings. This is also 

 arched as it breaks through the ground, and the curvature is 

 due, as in the hypocotyl, to the fact that the posterior side 



