the glands,—­by the glands absorbing various fluids or matter dissolved out of certain bodies,—­by exosmose,—­and by a certain degree of heat.  On the other hand, a temperature of about 150o Fahr. (65o.5 Cent.) does not excite aggregation; nor does the sudden crushing of a gland.  If a cell is ruptured, neither the exuded matter nor that which still remains within the cell undergoes aggregation when carbonate of ammonia is added.  A very strong solution of this salt and rather large bits of raw meat prevent the aggregated masses being well developed.  From these facts we may conclude that the protoplasmic fluid within a cell does not become aggregated unless it be in a living state, and only imperfectly if the cell has been injured.  We have also seen that the fluid must be in an oxygenated state, in order that the process of aggregation should travel from cell to cell at the proper rate.

Various nitrogenous organic fluids and salts of ammonia induce aggregation, but in different degrees and at very different rates.  Carbonate of ammonia is the most powerful of all known substances; the absorption of 1/134400 of a grain (.000482 mg.) by a gland suffices to cause all the cells of the same tentacle to become aggregated.  The first effect of the carbonate and of certain other salts of ammonia, as well as of some other fluids, is the darkening or blackening of the glands.  This follows even from long immersion in cold distilled water.  It apparently depends in chief part on the strong aggregation of their cell-contents, which thus become opaque, and do not reflect light.  Some other fluids render the glands of a brighter red; whilst certain acids, though much diluted, the poison of the cobra-snake, &c., make the glands perfectly white and opaque; and this seems to depend on the coagulation of their contents without [page 63] any aggregation.  Nevertheless, before being thus affected, they are able, at least in some cases, to excite aggregation in their own tentacles.

That the central glands, if irritated, send centrifugally some influence to the exterior glands, causing them to send back a centripetal influence inducing aggregation, is perhaps the most interesting fact given in this chapter.  But the whole process of aggregation is in itself a striking phenomenon.  Whenever the peripheral extremity of a nerve is touched or pressed, and a sensation is felt, it is believed that an invisible molecular change is sent from one end of the nerve to the other; but when a gland of Drosera is repeatedly touched or gently pressed, we can actually see a molecular change proceeding from the gland down the tentacle; though this change is probably of a very different nature from that in a nerve.  Finally, as so many and such widely different causes excite aggregation, it would appear that the living matter within the gland-cells is in so unstable a condition that almost any disturbance suffices to change its molecular nature, as in the case of certain chemical compounds.  And this change in the glands, whether excited directly, or indirectly by a stimulus received from other glands, is transmitted from cell to cell, causing granules of protoplasm either to be actually generated in the previously limpid fluid or to coalesce and thus to become visible.