United States or Cayman Islands ? Vote for the TOP Country of the Week !
The germination of the macrospore consists in the repeated division of its nucleus to form two groups of four, one group at each end of the embryo-sac. One nucleus from each group, the polar nucleus, passes to the centre of the sac, where the two fuse to form the so-called definitive nucleus. The three cells at the opposite end are known as antipodal cells and become invested with a cell-wall.
The development of the ovule, which represents the macrosporangium, is very similar to the process in Gymnosperms; when mature it consists of one or two coats surrounding the central nucellus, except at the apex where an opening, the micropyle, is left. The nucellus is a cellular tissue enveloping one large cell, the embryo-sac or macrospore.
In several natural orders and genera departures from the course of development just described have been noted. In the natural order Rosaceae, the series Querciflorae, and the very anomalous genus Casuarina and others, instead of a single macrospore a more or less extensive sporogenous tissue is formed, but only one cell proceeds to the formation of a functional female cell.
Thus, at a we see a macrospore, at b a fragment of the coat of a macrospore, at c another macrospore having a silicified nucleus, such as has been found in no other case, at d we have a transverse section of a vascular bundle, at e a longitudinal section of a rootlet traversed by another one, at f we have a transverse section of another rootlet, at g an almost entire portion of the vascular bundle of a root, and at h we see large pollen grains recalling those that we meet with in the silicified seeds from Saint Etienne.
Word Of The Day