Page:Miscellaneousbot01brow.djvu/566

 548 never observed the protruded thread of the Ovulum until after the secondary nucleus or Embryo, of which it is a continuation, becomes visible, I consider it as a production subsequent to impregnation.

It is possible, therefore, that the nearly similar tubes which have been observed terminating, as it is supposed, the nucleus of the imimpregnated Ovulum in a few other Families, may in some of these cases be of like origin.

2] To the observations formerly made on the general structure of Orchideae, I have here to add,—

1st, That the cells of the testa of the ripe seed are frequently spirally striated, though these cells in the Ovulum before and even for some time after impregnation are absolutely without striæ.

2nd, The Fibrillæ constituting the pubescence frequently produced, and in some cases entirely covering the surface of the aerial roots, as they have been called, of the parasitical portion of the Order, are very remarkable.

These Fibrillæ, which I have examined both in dried and recent specimens of several species, but more particularly in the living state in Renanthera coccinea, are simple tubular hairs without joints, and whose apices, by which they adhere when attached to other bodies, are either of the same diameter, or somewhat dilated; and then, as in Renanthera, often more or less lobed.

In their natural state they exhibit, in most cases, hardly any indication of spiral structure; but the membrane, of which they entirely consist, is sufficiently elastic to admit of being extended, and at the same time unrolled, to about twice the length of the Tube. They then form a broad ribbon of equal width throughout, and spirally twisted from right to left,—a direction opposite to that which generally obtains in spiral vessels. It is possible that this may not be the direction of the spire in all cases; it is manifest, however, very generally, if not universally, in Renanthera.

The existence of spiral tubes produced on the surface is probably of very rare occurrence; and among Phænogamous plants I have hitherto met with it only in the hairs