450 VEGETABLE RESPIRATION. NoTE XXXVII,

artery, is changed in colour from deep red to bright ſcarlet, and certainly in ſome of its efiential properties; it is then colle&ed by the pulmonary vein and returned to the heart. To ſhew a ſimilarity of circumſtances in the leaves of plants the following experiment was made, June 24, 1781. A ftalk with Icaves and ſeed-veflels of large ſpurge(Euphorbia helioſcopia) had been ſeveral days placed in a decottion of madder(Rubia tin&orum) ſo that the lower part of the ſtem, and two of the under- molt leaves were immerſed in it. After having waſhed the immerſed leaves in clear water, I could readily diſ- cern the colour of the madder paſſing along the middie rib of each leaß This red artery was beautifully viſible both on the under and upper ſurface of the leaf; but on the upper ſide many red branches were ſeen going from it to the extremities of the leaf, which on the other ſide were not viſible except by looking through it againſt the light. On this under ſide a ſyſtem of branching veßſlels carrying a pale milky fluid were ſeen coming from the extremities of the leaf, and covering the whole underſide of it, and joining into two large veins; one on each ſide of the red artery in the middle rib of the leaf, and along with it deſcending to the footſtalk or petiole. On ſitting „one of theſe leaves with ſciſlars, and having a common magnifying lens ready, the milky blood was ſeen oozing out of the returning veins on each ſide of the red artery in the middle rib, but none of the red fluid from the ar- tery. AI theſe appearances were more eaſily ſeen in a leaf of Picris treated in the ſame manner z for in this milky plant

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