45 A.M. Nov. 9th to 8.10 A.M. 11th. Movement of
bead magnified 6 times.
Allium cepa (Liliaceae).--The narrow green leaf, which protrudes from the
seed of the common onion as a cotyledon,* breaks through the ground in the
form of an arch, in the same manner as the hypocotyl or epicotyl of a
dicotyledonous plant. Long after the arch has risen above the surface the
apex remains within the seed-coats, evidently absorbing the still abundant
contents. The summit or crown of the arch, when it first protrudes from the
seed and is still buried beneath the ground, is simply rounded; but before
it reaches the surface it is developed into a conical protuberance of a
white colour (owing to the absence of chlorophyll), whilst the adjoining
parts are green, with the epidermis apparently rather thicker and tougher
than elsewhere. We may therefore conclude that this conical protuberance is
a special adaptation for breaking through the ground,** and answers the
same end as the knife-like white crest on the summit of the straight
cotyledon of the Gramineae.
* This is the expression used by Sachs in his 'Text-book of Botany.
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