25 inch, they sank
between 55o and 70o beneath the horizon. They acted, however, in this
manner only when they had been well illuminated during the day.
Nevertheless, the cotyledons have little or no power of bending towards a
lateral light, although the hypocotyl is strongly heliotropic. They are not
provided with a pulvinus, but continue to grow for a long time.
Ipomoea purpurea (vel Pharbitis hispida).--The cotyledons behave in all
respects like those of I. caerulea. A seedling with cotyledons .75 inch in
length (measured as before) and 1.65 inch in breadth, having a small true
leaf developed, was placed at 5.30 P.M. on a klinostat in a darkened box,
so that neither weight nor geotropism could act on them. At 10 P.M. one
cotyledon stood at 77o and the other at 82o beneath the horizon. Before
being placed in the klinostat they stood at 15o and 29o
[page 306]
beneath the horizon. The nocturnal position depends chiefly on the
curvature of the petiole close to the blade, but the whole petiole becomes
slightly curved downwards. It deserves notice that seedlings of this and
the last-named species were raised at the end of February and another lot
in the middle of March, and the cotyledons in neither case exhibited any
nyctitropic movement.
Pages:
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514