(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.]
From the foregoing experiments and observations we [page 389] see that
objects not containing soluble matter have little or no power of
exciting the glands to secrete. Non-nitrogenous fluids, if dense, cause
the glands to pour forth a large supply of viscid fluid, but this is
not in the least acid. On the other hand, the secretion from glands
excited by contact with nitrogenous solids or liquids is invariably
acid, and is so copious that it often runs down the leaves and collects
within the naturally incurved margins. The secretion in this state has
the power of quickly dissolving, that is of digesting, the muscles of
insects, meat, cartilage, albumen, fibrin, gelatine, and casein as it
exists in the curds of milk. The glands are strongly excited by
chemically prepared casein and gluten; but these substances (the latter
not having been soaked in weak hydrochloric acid) are only partially
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