(17) A particle of white sugar, placed on a leaf, formed in 1 hr. 10 m. a large drop of fluid, which in the course of 2 additional hours ran down into the naturally inflected margin.  This fluid was not in the least acid, and began to dry up, or more probably was absorbed, in 5 hrs. 30 m.  The experiment was repeated; particles being placed on a leaf, and others of the same size on a slip of glass in a moistened state; both being covered by a bell-glass.  This was done to see whether the increased amount of fluid on the leaves could be due to mere deliquescence; but this was proved not to be the case.  The particle on the leaf caused so much secretion that in the course of 4 hrs. it ran down across two-thirds of the leaf.  After 8 hrs. the leaf, which was concave, was actually filled with very viscid [page 388] fluid; and it particularly deserves notice that this, as on the former occasion, was not in the least acid.  This great amount of secretion may be attributed to exosmose.  The glands which had been covered for 24 hrs. by this fluid did not differ, when examined under the microscope, from others on the same leaf, which had not come into contact with it.  This is an interesting fact in contrast with the invariably aggregated condition of glands which have been bathed by the secretion, when holding animal matter in solution.

(18) Two particles of gum arabic were placed on a leaf, and they certainly caused in 1 hr. 20 m. a slight increase of secretion.  This continued to increase for the next 5 hrs., that is for as long a time as the leaf was observed.

(19) Six small particles of dry starch of commerce were placed on a leaf, and one of these caused some secretion in 1 hr. 15 m., and the others in from 8 hrs. to 9 hrs.  The glands which had thus been excited to secrete soon became dry, and did not begin to secrete again until the sixth day.  A larger bit of starch was then placed on a leaf, and no secretion was excited in 5 hrs. 30 m.; but after 8 hrs. there was a considerable supply, which increased so much in 24 hrs. as to run down the leaf to the distance of 3/4 of an inch.  This secretion, though so abundant, was not in the least acid.  As it was so copiously excited, and as seeds not rarely adhere to the leaves of naturally growing plants, it occurred to me that the glands might perhaps have the power of secreting a ferment, like ptyaline, capable of dissolving starch; so I carefully observed the above six small particles during several days, but they did not seem in the least reduced in bulk.  A particle was also left for two days in a little pool of secretion, which had run down from a piece of spinach leaf; but although the particle was so minute no diminution was perceptible.  We may therefore conclude that the secretion cannot dissolve starch.  The increase caused by this substance may, I presume, be attributed to exosmose.  But I am surprised that starch acted so quickly and powerfully as it did, though in a less degree than sugar.  Colloids are known to possess some slight power of dialysis; and on placing the leaves of a Primula in water, and others in syrup and diffused starch, those in the starch became flaccid, but to a less degree and at a much slower rate than the leaves in the syrup; those in water remaining all the time crisp.]