CHAP. III. THE PROCESS OF AGGREGATION. 63 



any aggregation. Nevertheless, before being thug 

 affected, they are able, at least in some cases, to excite 

 aggregation in their own tentacles. 



That the central glands, if irritated, send centri- 

 fugally 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 aggrega- 

 tion 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 Aggre- 

 gation in the Hoots of Plants. 



It will hereafter be seen that a weak solution ol the tvu- 

 bonate 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 1 met with, viz. Euphorbia peplus, being care- 



