Chap. U. BEEAKING THROUGH THE GROUND. 93 



80 mm. on Sachs' cyclometer) in the same relative 

 direction as shown at B in Tig. 59. As geotropism 

 will obviously tend to check this curvature, seven 

 seeds were allowed to germinate with proper pre- 

 cautions for their growth in a klinostat,* by which 

 means geotropism was eliminated. The position of the 

 hypocotyls was observed during four successive days, 

 and they continued to bend towards the hilum and 

 lower surface of the seed. On the fourth day they 

 were deflected by an average angle of 63° from a lino 

 perpendicular to the lower surface, and were therefore 

 considerably more curved than the hypocotyl and 

 radicle in the bean at B (Fig. 59), though in the same 

 relative direction. 



It will, we presume, be admitted that all leguminous 

 plants with hypogean cotyledons are descended from 

 forms which once raised their cotyledons above the 

 ground in the ordinary manner ; and in doing so, it is 

 certain that their hypocotyls would have been abruptly 

 arched, as in the case of every other dicotyledonous 

 plant. This is especially clear in the case of Phaseolus, 

 for out of five species, the seedlings of which we 

 observed, namely, P. muUiflorus, caracalla, vulgaris, 

 Hernandesii and Boxburghii (inhabitants of the Old 

 and New Worlds), the three last-named species have 

 well-developed hypocotyls which break through the 

 ground as arches. Now, if we imagine a seedling of 

 the common bean or of P. muUiflorus, to behave as its 

 progenitors once did, the hypocotyl (h, Fig. 59), in 

 whatever position the seed may have been buried, 

 would become so much arched that the upper part 

 would be doubled down parallel to the lower part ; and 



• An instrument devised by on which the plant under observfi- 

 Sachs, consisting essentially of a tion is supported : see ' Wurzburg 

 ■lowly revolving horizontal axis, Aibeiten,' 1879, p. 209. 



