A HISTORY OF SCIENCE 



and returned to the heart. To show a similarity of 

 circumstances in the leaves of plants, the following ex- 

 periment was made, June 24, 1781. A stalk with 

 leaves and seed-vessels of large spurge (Euphorbia 

 helioscopia) had been several days placed in a decoction 

 of madder (Rubia tinctorum) so that the lower part of 

 the stem and two of the undermost leaves were im- 

 mersed in it. After having washed the immersed 

 leaves in clear water I could readily discover the color 

 of the madder passing along the middle rib of each leaf. 

 The red artery was beautifully visible on the under and 

 on the upper surface of the leaf ; but on the upper side 

 many red branches were seen going from it to the ex- 

 tremities 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 vessels carry- 

 ing a pale milky fluid were seen coming from the ex- 

 tremities of the leaf, and covering the whole under side 

 of it, and joining 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 foot-stalk or petiole. On 

 slitting one of these leaves with scissors, and having a 

 magnifying-glass ready, the milky blood was seen ooz- 

 ing out of the returning veins on each side of the red 

 artery in the middle rib, but none of the red fluid 

 from the artery. 



''All these appearances were more easily seen in a 

 leaf of Picris treated in the same manner; for in this 

 milky plant the stems and middle rib of the leaves are 

 sometimes naturally colored reddish, and hence the 

 color of the madder seemed to pass farther into the 

 ramifications of their leaf -arteries, and was there beau- 



