90 HYPOCOTYLS, EPICOTYLS, ETC., CHAP. II. 



had done when the seeds were first buried. But it may 

 be argued in this and other such cases that, as the 

 hypocotyl grows up through the soil, the seed will 

 almost certainly be tilted to one side ; and then 

 from the resistance w r hich it must offer during its 

 further elevation, the upper part of the hypocotyl will 

 be doubled down and thus become arched. This view 

 seems the more probable, because with Ranunculus 

 ficaria only the petioles of the leaves which forced 

 a passage through the earth were arched ; and not 

 those which arose from the summits of the bulbs above 

 the ground. Nevertheless, this explanation does not 

 apply to the Cucurbita, for when germinating seeds 

 were suspended in damp air in various positions by 

 pins passing through the cotyledons, fixed to the 

 inside of the lids of jars, in which case the hypo- 

 cotyls were not subjected to any friction or constraint, 

 yet the upper part became spontaneously arched. 

 This fact, moreover, proves that it is not the weight 

 of the cotyledons which causes the arching. Seeds 

 of Helianthus annum and of two species of Ipomoaa 

 (those of I. lona nox being for the genus large 

 and heavy) were pinned in the same manner, 

 and the hypocotyls became spontaneously arched ; 

 the radicles, which had been vertically dependent, 

 assumed in consequence a horizontal position. In 

 the case of Ipomcea leptophylla it is the petioles of the 

 cotyledons which become arched whilst rising through 

 the ground ; and this occurred spontaneously when 

 the seeds were fixed to the lids of jars. 



It may, however, be suggested with some degree of 

 probability that the arching was aboriginally caused 

 by mechanical compulsion, owing to the confinement 

 of the parts in question within the seed-coats, or to 

 friction whilst they were being dragged upwards. But 



