284: PRINCIPLES OF BOTANY, ETC. 
during warm weather, but descends again when cold 
supervenes. Others only allow the ascent of the sap 
and its transpiration through the pores of the cutis, 
but deny its descent or reflux, as this, they believe, 
would hurt the structure of the plant. 
Malpighi was the first who ascribed irritability to 
the smaller vessels, and supposed that they were 
sometimes contracted, sometimes dilated in diameter. 
This philosopher even asserts, that in one of the air 
vessels he actually observed a peristaltic motion of 
its spiral windings, similar to that of the animal in- 
testines. But was he not deceived by the elasticity 
of the twisted vessels, which to see them distinctly 
must be separated 
Brugmanns confirmed this irritability of slants 
which Malpighi only suspected, by a series of ele- 
gant experiments. Branches of the Euphorbia /a- 
thyris and myrsinites, when cut off, discharged a con- 
siderable quantity of milk-like fluid out of their ves- 
sels. ‘Chis haemorrhage he stopped immediately by 
a solution of alum and sulphat of iron, which was 
so diluted as not in the least to stain paper or linen. 
The stoppage of the flow of the juice is certainly to 
be ascribed only to the solution of the alum and 
sulphat of iron, contracting the apertures of the 
vessels. Van Marum repeated this experiment, but 
without the same result. . It is indeed put beyond 
doubt, that the propulsion of the sap depends on the 
peculiar contraction and dilatation of the vessels, not 
on capillary attraction, nor on the rarification of the 
air by means of the solar rays. Even Bonnet him- 
self, who at first adhered strictly to Hales’s opinion, 
found 
