Cuar. II] THE PROCESS OF AGGREGATION. 53 
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 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 pheno- 
menon. 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 tothe 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. 
Supplementary Observations on the Process of Aggregation 
in the Roots of Plants. 
Jt will hereafter be seen that a weak solution of the carbonate of 
ammonia induces aggregation in the cells of the roots of Drosera; and 
this led me to make a few trials on the roots of other plants. I dug 
up in the latter part of October the first weed which I met with, viz. 
* (The words “which .... light ” would probably have been omitted 
by the author in a second edition. —F. D.] 
