510 ON THE ORGANS AND MODE OP FECUNDATION
the pollen of one species may be excited by the stigma of another belonging to a very different tribe.
The elongation of the tubes, so remarkable in this family, and their separation from the grain long before their growth is completed, render it probable that they derive nourish- ment either from the particles contained in the grain, or from the conducting surfaces with which they are in contact.
The first visible effect of the action of the pollen on the stigma is the enlargement of the ovarium, which, in cases where it was reversed by torsion in the flowering state, generally untwists and resumes its original position.
Of the changes produced in the ovulum consequent to impregnation, the first consists in its enlargement merely ; and in the few cases where the nucleus is at this period still partially exposed, it becomes completely covered by the testa, the original apex, but now the lower extremity of which continues open. The next change consists in the disappearance of the nucleus, probably from its acquiring greater transparency, and becoming confluent with the substance of the testa. Soon after, or perhaps simulta- neously with, the disappearance of the original nucleus, and Avhile the enlargement of the whole ovulum is gradually proceeding, a minute opaque round speck, generally seated about the middle of the testa, becomes visible. The 709] opaque speck is the commencement of the future embryo. At this period, or until the opaque corpuscle or nucleus has acquired more than half the size it attains in the ripe seed, a thread may be traced from its apex very nearly to the open end of the testa, or as it may be supposed, to the apex of the original nucleus of the unimpregnated ovulum.
This thread consists of a simple series of short cells, in one of which, in a single instance only however, I observed a circulation of very minute granular matter ; and in seve- ral cases I have been able to distinguish in these cells that granular areola so frequently existing in the cells of Orchideous plants, and to which I shall have occasion here- after to advert.
The lowermost joint or cell of this thread is probably the original state of what afterwards, from enlargement and
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