IX ORCIIIDE/E AND ASCLEPIADE.E. 509
ovuliiui. ^l. Dii Petit Tliouars also, in his account already quoted of these cords, suj)|)osed by hiiu to belong; to the stigma of OrchidetC, describes their ultimate ramifications as mixing with the ovula.
I do not however consider myself so far advanced as these observers in this very important point ;^ and what I shall have to adduce on the subject of Asclepiadeae, makes me hesitate still more to adopt their statements.
I may also remark that in OrchidccC the six cords are to be met with even in the ]'i})e capsule, in which, allow- ance being made for the effect of pressure, they are not materially reduced in size; and the statement by M. Du Petit Thouars, of the lateral branches separating the ovula into irregular groups, is certainly not altogether correct ; these groups behig equally distinct before the existence of the cords.
With regard to the question of the origin of the pollen tubes, several arguments might be adduced in favour of ^1. Brongniart's opinion ; which is, that they belong to the inner mendjrane of the grain, the intimate cohesion of the two membranes being assumed in most cases, and the no less intimate union of the constituent parts of compound gniins in some others. That an inner mcmbra}ie does oc- casionally exist is naanifest in the pollen of several Coniferae, in which the outer coat regularly bursts and is deciduous ; and it will hereafter appear, that the structure in Ascle- piadca^ contirms the correctness of this view.
But whatever opinion may be entertained as to the [rrs origin of the tube, it can hardly be questioned that its pro- duction or growth is a vital action excited in the grain by the application of an external stimulus. The appropriate and most powerful stinudus to this action is no doubt con- tact, at the proper period, with the secretion or surface of the stigma of the same species. Many facts, however, and among others the existence of hybrid plants, j)rove that this is not the only stimulus capable of ])roducing the effect ; and in Orchidex' I have found that the action in
' Sec Additional Observatious.
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