the Vasa propria, and Receptacles of the Juices of Plants. 407 



The remotest venules are accompanied by fine vascular branches; 

 for in the course of division the latter become more and more 

 attenuated. This circumstance is observable in Ficus and Che- 

 lidonium. 



The vasa propria of stems appertain especially to the cortical 

 system. Thus, in Papaver and Lactuca, the special juices are 

 seen, on section, not to flow from the central medulla ; or if they 

 do so at all, it is only in very minute quantity. Nevertheless 

 in some other plants, as Campanula and Chelidonium majus, 

 very many such special vessels occur in the woody lamina ; and 

 there are plants, indeed, in which such structures are more 

 abundant in the medulla than in the cortical zones; among 

 such is Asclepias syriaca. The vessels of stems may be detached 

 in considerable numbers, and isolated by maceration after pre- 

 vious boiling. The proper vessels of the cortex are distributed 

 in the different tissues of its layers : thus, in Campanula they 

 are diffused in the parenchyma and in the fibrous layer of the 

 bark ; in Chelidonium they lie outside the fibrous bundles. The 

 vasa propria of stems are generally but slightly ramified, though 

 not, indeed, devoid of frequent divisions. In Asclepias these 

 vessels anastomose at every node, in such a manner, too, as to 

 form a plexus and a kind of septum in the medulla. Some of the 

 branches emanating from this plexus are continued to the petiole 

 of the leaf at its junction, with a node, and to the young branch 

 which springs from its axil ; in this way they traverse the me- 

 dullary space left between the bundles of woody fibre, and anas- 

 tomose with the vasa propria of the bark, thus establishing a 

 communication between the vascular network of the medulla 

 and that of the cortical system. 



The proper juices are in general more dense and of a deeper 

 colour in the lower and older parts of a plant. In young shoots 

 they are pale, and not thick ; towards the base of the stem they 

 are habitually much more intense in colour. Thus, in Asclepias 

 syriaca, the juices, which are of a pure white colour in the upper 

 portions of the plant, are of a yellow colour near the base of 

 its stem. In Chelidonium the juices at the extremities of the 

 branches are of a very pale yellow, but of a deep yellow tint in 

 the main stem, and a reddish yellow in the root. 



These dispositions are reversed, however, in certain species : 

 in Papaver the proper juices are of a milky-white colour and 

 well marked in the capsules, though scarcely opaline in the root. 

 The proper juices of this plant seem to be derived principally 

 from the fruit, which gives off a white juice on incision in great 

 abundance : yet if the petiole be cut, little exudes ; and if the 

 incision be low down in the stem, no escape at all will probably 

 take place. 



