CIRCULATION. 103 



These vessels exist in most monocotyledonous and dicoty- 

 ledonous plants. We find them generally in those vascular 

 bundles which form the salient lines on the under surface 

 of leaves, designated as nervures. They are also found in 

 the interior bark. In monocotyledons, we meet with them 

 in the vascular bundles developed in the midst of the cel- 

 lular tissue which forms the mass of the stem. 



The latex itself is usually a fluid of a white, yellow, or 

 reddish color. This color is due to the presence of opaque 

 corpuscles of various hues, which being collected together 

 in abundance in a watery and transparent liquid, commu- 

 nicate to it their color, just as the blood and milk-globules 

 give to those liquids, colorless in themselves, the red and 

 white colors by which they are characterized. The latex 

 when poured out by itself into a vessel, behaves like the 

 blood, and separates into two parts ; a liquid, colorless or 

 slightly colored brown, and a solid matter forming a sort 

 of clot, composed of colored globules. These globules con- 

 sist of various matters insoluble in water, such as wax, fatty 

 matters, and caoutchouc. 



The motion of the latex or milk-sap is seen in a young 

 expanded leaf, it may be in a sepal or a petal, still adhering 

 to the plant, especially when it contains a considerable 

 quantity of colored sap, as for instance, the young ex- 

 panded sepal of Chelidonium majus, or the large stipule 

 which encloses the terminal bud of Ficus elastica. If the epi- 

 dermis be carefully raised without injuring the vessels, the 

 nature of the movement will be perceived. Similar move- 

 ments probably take place in the anastomosing capillary 

 vessels of all plants, but they are rendered impercep- 

 tible to us by the transparency of their juices. 



