Fig. 52. Zea mays: track left on inclined smoked glass-plate by tip of
radicle in growing downwards.
Avena sativa (Gramineae).--A cotyledon, 1 ? inch in height, was placed in
front of a north-east window, and the movement of the apex was traced on a
horizontal glass during two days. It moved towards the light in a slightly
zigzag line from 9 to 11.30 A.M. on October 15th; it then moved a little
backwards and zigzagged much until 5 P.M., after which hour, and curing the
night, it continued to move towards the window. On the following morning
the same movement was continued in a nearly straight line until 12.40 P.M.,
when the sky remained until 2.35 extraordinarily dark from thunder-clouds.
During this interval of 1 h. 55 m., whilst the light was obscure, it was
interesting to observe how circumnutation overcame heliotropism, for the
apex, instead of continuing to move towards the window in a slightly zigzag
line, reversed its course four times, making two small narrow ellipses. A
diagram of this case will be given in the chapter on Heliotropism.
[page 66]
A filament was next fixed to a cotyledon only 1/4 of an inch in height,
which was illuminated exclusively from above, and as it was kept in a warm
greenhouse, it grew rapidly; and now there could be no doubt about its
circumnutation, for it described a figure of 8 as well as two small
ellipses in 5 ? hours.
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