12 CIRCUMNUTATION OF SEEDLINGS. CHAP. I. 



'2nd in the same general direction, and in a similar zigzag 

 manner. From the radicle not being quite perpendicular when 

 the filament was affixed geotropism came into play at once; 

 but the irregular zigzag course shows that there was growth 

 (probably preceded by turgescence), sometimes on one and 

 sometimes on another side. Occasionally the bead remained 

 stationary for about an hour, and then probably growth occurred 

 on the side opposite to that which caused the geotropic curva- 

 ture. In the case previously described the basal part of the 

 very short radicle from being turned vertically upwards, was at 

 first very little affected by geotropism. Filaments were affixed 

 in two other instances to rather longer radicles protruding 

 obliquely from seeds which had been turned upside down ; and 

 in these cases the lines traced on the horizontal glasses were 

 only slightly zigzag, and the movement was always in the same 

 general direction, through the action of geotropism. All these 

 observations are liable to several causes of error, but we believe, 

 from what will hereafter be shown with respect to the move- 

 ments of the radicles of other plants, that they may be largely 

 trusted. 



Hypocotyl. The hypocotyl protrudes through the seed-coats 

 as a rectangular projection, which grows rapidly into, an arch 

 like the letter U turned upside down f] 5 the cotyledons being 

 still enclosed within the seed. In whatever position the seed 

 may be embedded in the earth or otherwise fixed, both legs of 

 the arch bend upwards through apogeotropism, and thus rise 

 vertically above the ground. As soon as this has taken place, 

 or even earlier, the inner or concave surface of the arch grows 

 more quickly than the upper or convex surface; and this tends 

 to separate the two legs and aids in drawing the cotyledons out 

 of the buried seed-coats. By the growth of the whole arch the 

 cotyledons are ultimately dragged from beneath the ground, even 

 from, a considerable depth; and now the hypocotyl quickly 

 straightens itself by the increased growth of the concave side. 



Even whilst the arched or doubled hypocotyl is still beneath 

 the ground, it circumnutates as much as the pressure of the sur- 

 rounding soil will permit; but this was difficult to observe, 

 because as soon as the arch is freed from lateral pressure the two 

 legs begin to separate, even at a very early age, before the arch 

 would naturally have reached the surface. Seeds were allowed 

 to germinate on the surface of damp earth, and after they had 

 fixed themselves by their radicles, and after the, as yet, only 



