There has been much discussion by botanists on the homological nature of these projections.  Dr. Nitschke (’Bot.  Zeitung,’ 1861, p. 146) believes that they correspond with the fimbriated scale-like bodies found at the bases of the petioles of Drosera. [page 323]

is slightly concave, and is formed, according to Cohn, of three layers of cells.  Its upper surface is studded with colourless glands like, but more simple than, those of Dionaea; they are supported on distinct footstalks, consisting of two rows of cells.  The outer

Fig. 13. (Aldrovanda vesiculosa.) Upper figure, whorl of leaves (from Prof.  Cohn).  Lower figure, leaf pressed flat open and greatly enlarged.

and broader portion of the lobe is flat and very thin, being formed of only two layers of cells.  Its upper surface does not bear any glands, but, in their place, small quadrifid processes, each consisting of four tapering projections, which rise from a common [page 324] prominence.  These processes are formed of very delicate membrane lined with a layer of protoplasm; and they sometimes contain aggregated globules of hyaline matter.  Two of the slightly diverging arms are directed towards the circumference, and two towards the midrib, forming together a sort of Greek cross.  Occasionally two of the arms are replaced by one, and then the projection is trifid.  We shall see in a future chapter that these projections curiously resemble those found within the bladders of Utricularia, more especially of Utricularia montana, although this genus is not related to Aldrovanda.

A narrow rim of the broad flat exterior part of each lobe is turned inwards, so that, when the lobes are closed, the exterior surfaces of the infolded portions come into contact.  The edge itself bears a row of conical, flattened, transparent points with broad bases, like the prickles on the stem of a bramble or Rubus.  As the rim is infolded, these points are directed towards the midrib, and they appear at first as if they were adapted to prevent the escape of prey; but this can hardly be their chief function, for they are composed of very delicate and highly flexible membrane, which can be easily bent or quite doubled back without being cracked.  Nevertheless, the infolded rims, together with the points, must somewhat interfere with the retrograde movement of any small creature, as soon as the lobes begin to close.  The circumferential part of the leaf of Aldrovanda thus differs greatly from that of Dionaea; nor can the points on the rim be considered as homologous with the spikes round the leaves of Dionaea, as these latter are prolongations of the blade, and not mere epidermic productions.  They appear also to serve for a widely different purpose. [page 325]