Summary and Concluding Remarks.—­The process of aggregation is independent of the inflection of the tentacles and of increased secretion from the glands.  It commences within the glands, whether these have been directly excited, or indirectly by a stimulus received from other glands.  In both cases the process is transmitted from cell to cell down the whole length of the tentacles, being arrested for a short time at each transverse partition.  With pale-coloured leaves the first change which is perceptible, but only

* With respect to plants, Sachs, ‘Trait de Bot.’ 3rd edit., 1874, p. 864.  On blood corpuscles, see ’Quarterly Journal of Microscopical Science,’ April 1874, p. 185.’ [page 61]

under a high power, is the appearance of the finest granules in the fluid within the cells, making it slightly cloudy.  These granules soon aggregate into small globular masses.  I have seen a cloud of this kind appear in 10 s. after a drop of a solution of carbonate of ammonia had been given to a gland.  With dark red leaves the first visible change often is the conversion of the outer layer of the fluid within the cells into bag-like masses.  The aggregated masses, however they may have been developed, incessantly change their forms and positions.  They are not filled with fluid, but are solid to their centres.  Ultimately the colourless granules in the protoplasm which flows round the walls coalesce with the central spheres or masses; but there is still a current of limpid fluid flowing within the cells.  As soon as the tentacles fully re-expand, the aggregated masses are redissolved, and the cells become filled with homogeneous purple fluid, as they were at first.  The process of redissolution commences at the bases of the tentacles, thence proceeding upwards to the glands; and, therefore, in a reversed direction to that of aggregation.

Aggregation is excited by the most diversified causes,—­by the glands being several times touched,—­by the pressure of particles of any kind, and as these are supported by the dense secretion, they can hardly press on the glands with the weight of a millionth of a grain,*—­by the tentacles being cut off close beneath

* According to Hofmeister (as quoted by Sachs, ‘Trait de Bot.’ 1874, p. 958), very slight pressure on the cell-membrane arrests immediately the movements of the protoplasm, and even determines its separation from the walls.  But the process of aggregation is a different phenomenon, as it relates to the contents of the cells, and only secondarily to the layer of protoplasm which flows along the walls; though no doubt the effects of pressure or of a touch on the outside must be transmitted through this layer. [page 62]