Chap. III.] THE PROCESS OF AGGREGATION. 45 



stimulant, and even small bits generally injure, and sometimes 

 kill, the leaves to which they are given: the aggregated masses 

 of protoplasm become dingy or almost colourless, and present an 

 unusual granular appearance, as is likewise the case with leaves 

 which have been iraniei-sed in a very strong solution of carbonate 

 of ammonia. A leaf placed in milk had the contents of its cells 

 somewhat aggregated in 1 hr. Two other leaves, one immersed in 

 human saliva for 2 hrs. 30 m., and another in unboiled white of egg 

 for 1 hr. 30 m., were not acted on in this manner; though they 

 undoubtedly would have been so, had more time been allowed. 

 These same two leaves, on being afterwards placed in a solution of 

 carbonate of ammonia (3 grs. to 1 oz.), had their cells aggregated, 

 the one in 10 m. and the other in 5 m. 



Several leaves were left for 4 hrs. 30 m. in a solution of one part 

 of white sugar to 146 of water, and no aggregation ensued; on 

 being placed in a solution of this same strength of carbonate of 

 ammonia, they were acted on in 5 m. ; as was likewise a leaf which 

 had been left for 1 hr. 45 m. in a moderately thick solution of gum 

 arable. Several other leaves were immersed for some hours in 

 denser solutions of sugar, gum, and starch, and they had the con- 

 tents of their cells greatly aggregated. This effect may be attrib- 

 uted to exosmose; for the leaves in the syrup became quite flac- 

 cid, and those in the gum and starch somewhat flaccid, with their 

 tentacles twisted about in the most irregular manner, the longer 

 ones like corkscrews. We shall hereafter see that solutions of 

 these substances, when placed on the discs of leaves, do not excite 

 inflection. Particles of soft sugar were added to the secretion 

 round several glands and were soon dissolved, causing a great in- 

 crease of the secretion, no doubt by exosmose; and after 24 hrs. 

 the cells showed a certain amount of aggregation, though the ten- 

 tacles were not inflected. Glycerine causes in a few minutes well- 

 pronounced aggregation, commencing as usual within the glands 

 and then travelling down the tentacles; and this I presume may be 

 attributed to the strong attraction of this substance for water. 

 Immersion for several hours in water causes some degree of aggre- 

 gation. Twenty leaves were first carefully examined, and re- 

 examined after having been left immersed in distilled water for 

 various periods, with the following results. It is rare to find even 

 a trace of aggregation until 4 or 5 and generally not until several 

 more hours have elapsed. When, however, a leaf becomes quickly 

 inflected in water, as sometimes happens, especially during very 

 warm weather, aggregation may occur in little over 1 hr. In all 

 cases leaves left in water for more than 24 hrs. have their glands 

 blackened, which shows that their contents are aggregated ; and in 

 the specimens, which were carefully examined, there was fairly 

 well-marked aggregation in the upper cells of the pedicels. These 

 trials were made with cut-ofT leaves, and it occurred to mo that 

 this circumstance might influence the result, as the footstalks 

 would not perhaps absorb water quickly enough to supply the 

 glands as they continued to secrete. But this view was proved 

 erroneous, for a plant with uninjured roots, bearing four leaves. 



