of water (i.e. one part to 87,500) is here reproduced: such energetic inflection is never caused by water alone. With leaves in the weak solutions, the blade or lamina often becomes inflected; and this is so rare a circumstance with leaves in water that I have seen only two instances; and in both of these the inflection was very feeble. Again, with leaves in the weak solutions, the inflection of the tentacles and blade often goes on steadily, though slowly, increasing during many hours; and [page 141] this again is so rare a circumstance with leaves in water that I have seen only three instances of any such increase after the first 8 to 12 hrs.; and in these three instances the two outer rows of tentacles were not at all affected. Hence there is sometimes a much greater difference between the leaves in water and in the weak solutions, after from 8 hrs. to 24 hrs., than there was within the first 3 hrs.; though as a general rule it is best to trust to the difference observed within the shorter time.
With respect to the period of the re-expansion of the leaves, when left immersed either in water or in the weak solutions, nothing could be more variable. In both cases the exterior tentacles not rarely begin to re-expand, after an interval of only from 6 to 8 hrs.; that is just about the time when the short tentacles round the borders of the disc become inflected. On the other hand, the tentacles sometimes remain inflected for a whole day, or even two days; but as a general rule they remain inflected for a longer period in very weak solutions than in water. In solutions which are not extremely weak, they never re-expand within nearly so short a period as six or eight hours. From these statements it might be thought difficult to distinguish between the effects of water and the weaker solutions; but in truth there is not the slightest difficulty until excessively weak solutions are tried; and then the distinction, as might be expected, becomes very doubtful, and at last disappears. But as in all, except the simplest, cases the state of the leaves simultaneously immersed for an equal length of time in water and in the solutions will be described, the reader can judge for himself.]
CARBONATE OF AMMONIA.
This salt, when absorbed by the roots, does not cause the tentacles to be inflected. A plant was so placed in a solution of one part of the carbonate to 146 of water that the young uninjured roots could be observed. The terminal cells, which were of a pink colour, instantly became colourless, and their limpid contents cloudy, like a mezzo-tinto engraving, so that some degree of aggregation was almost instantly caused; but no further change ensued, and the absorbent hairs were not visibly affected. The tentacles [page 142] did not bend. Two other plants were placed with their roots surrounded by damp moss, in half an ounce (14.198 ml.) of a solution of one part of the carbonate to 218 of water, and were observed for 24 hrs.; but not a single tentacle was inflected. In order to produce this effect, the carbonate must be absorbed by the glands.
The vapour produces a powerful effect on the glands, and induces inflection. Three plants with their roots in bottles, so that the surrounding air could not have become very humid, were placed under a bell-glass (holding 122 fluid ounces), together with 4 grains of carbonate of ammonia in a watch-glass. After an interval of 6 hrs. 15 m. the leaves appeared unaffected; but next morning, after 20 hrs., the blackened glands were secreting copiously, and most of the tentacles were strongly inflected. These plants soon died. Two other plants were placed under the same bell-glass, together with half a grain of the carbonate, the air being rendered as damp as possible; and in 2 hrs. most of the leaves were affected, many of the glands being blackened and the tentacles inflected. But it is a curious fact that some of the closely adjoining tentacles on the same leaf, both on the disc and round the margins, were much, and some, apparently, not in the least affected.