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. Supplementary Observations on the Process of Aggregation in the Roots of Plants. It 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. Euphorbia peplus, being care- [page 64] ful not to injure the roots; these were washed and placed in a little solution of one part of carbonate of ammonia to 146 of water. In less than one minute I saw a cloud travelling from cell to cell up the roots, with wonderful rapidity. After from 8 m. to 9 m. the fine granules, which caused this cloudy appearance, became aggregated towards the extremities of the roots into quadrangular masses of brown matter; and some of these soon changed their forms and became spherical. Some of the cells, however, remained unaffected. I repeated the experiment with another plant of the same species, but before I could get the specimen into focus under the microscope, clouds of granules and quadrangular masses of reddish and brown matter were formed, and had run far up all the roots. A fresh root was now left for 18 hrs. in a drachm of a solution of one part of the carbonate to 437 of water, so that it received 1/8 of a grain, or 2.024 mg. When examined, the cells of all the roots throughout their whole length contained aggregated masses of reddish and brown matter. Before making these experiments, several roots were closely examined, and not a trace of the cloudy appearance or of the granular masses could be seen in any of them. Roots were also immersed for 35 m. in a