Although in innumerable instances foliar organs move when excited, no case is known of a trichome having such [page 359] power.* We are thus led to inquire how the so-called tentacles of Drosera, which are manifestly of the same general nature as the glandular hairs of the above three genera, could have acquired the power of moving.  Many botanists maintain that these tentacles consist of prolongations of the leaf, because they include vascular tissue, but this can no longer be considered as a trustworthy distinction.  The possession of the power of movement on excitement would have been safer evidence.  But when we consider the vast number of the tentacles on both surfaces of the leaves of Drosophyllum, and on the upper surface of the leaves of Drosera, it seems scarcely possible that each tentacle could have aboriginally existed as a prolongation of the leaf.  Roridula, perhaps, shows us how we may reconcile these difficulties with respect to the homological nature of the tentacles.  The lateral divisions of the leaves of this plant terminate in long tentacles; and these include spiral vessels which extend for only a short distance up them, with no line of demarcation between what is plainly the prolongation of the leaf and the pedicel of a glandular hair.  Therefore there would be nothing anomalous or unusual in the basal parts of these tentacles, which correspond with the marginal ones of Drosera, acquiring the power of movement; and we know that in Drosera it is only the lower part which becomes inflected.  But in order to understand how in this latter genus not only the marginal but all the inner tentacles have become capable of movement, we must further assume, either that through the principle of correlated development this

* Sachs, ‘Trait de Botanique’ 3rd edit. 1874, p. 1026.

  Dr. Warming ‘Sur la Diffrence entres les Trichomes,’ Copenhague,
  1873, p. 6.  ’Extrait des Videnskabelige Meddelelser de la Soc.
d’Hist. nat. de Copenhague,’ Nos. 10-12, 1872. [page 360]

power was transferred to the basal parts of the hairs, or that the surface of the leaf has been prolonged upwards at numerous points, so as to unite with the hairs, thus forming the bases of the inner tentacles.

The above named three genera, namely Drosophyllum, Roridula, and Byblis, which appear to have retained a primordial condition, still bear glandular hairs on both surfaces of their leaves; but those on the lower surface have since disappeared in the more highly developed genera, with the partial exception of one species, Drosera binata.  The small sessile glands have also disappeared in some of the genera, being replaced in Roridula by hairs, and in most species of Drosera by absorbent papillae.  Drosera binata, with its linear and bifurcating leaves, is in an intermediate condition.  It still bears some sessile glands on both surfaces of the leaves, and on the lower surface a few irregularly placed tentacles, which are incapable of movement.  A further slight change would convert the linear leaves of this latter species into the oblong leaves of Drosera anglica, and these might easily pass into orbicular ones with footstalks, like those of Drosera rotundifolia.  The footstalks of this latter species bear multicellular hairs, which we have good reason to believe represent aborted tentacles.