740 PHYSIOLOGY. 



The clown current of elaborated sap is generally believed to pass 

 through the long liber-cells of the inner bark. 



3. Functions of Vessels and Vascular Tissue. — The func- 

 tions of the spiral, annular, reticulated, pitted, and scalariform 

 vessels have been a subject of much dispute from an early period, 

 and have been repeatedly investigated. Hales, BischofF, and others 

 came to the conclusion that these vessels were carriers of air, and 

 it is certain that air only is found in old vessels ; while Dutrochet, 

 Link, Kominger, &c., believed that their essential function was 

 to carry fluids from the root upwards. According to Link, when 

 plants are watered for several days with a solution of ferro- 

 cyanide of potassium, and afterwards with a solution of per- 

 sulphate of iron, prussian blue is found in the vessels, and not 

 in the prosenchymatous cells, as the experiments of Hoffman, 

 alluded to in speaking of the functions of prosenchymatous cells, 

 seem to indicate (see p. 739) ; and, more recently, the experiments 

 of Herbert Spencer, conducted with great care, tend to show that 

 in young plants at all events the vessels are the chief sap-car- 

 riers, whence the fluid exudes into the surrounding prosenchyma. 



Functions of Latieiferous Vessels or Tissues. — The physio- 

 logical importance of these vessels has given rise to much 

 discussion, and is at the present time involved in obscurity. 

 Nothing further is absolutely known, than that they contain a 

 watery granular fluid which becomes milky on exposure to air, 

 and to which the name of latex has been given. (See p. 45.) 

 Lindley, and some other authors, believe that they " convey the 

 elaborated sap of a plant to the places where it is needed, and 

 especially down the inner part of the bark of Exogens." 

 Schultz called the tissue formed by the ramifications of the 

 latieiferous vessels cim nchymei, because he believed that he had 

 discovered in it a peculiar vital movement or circulation of the 

 latex, to which he gave the name of en/closis. Lestiboudois, as 

 already noticed (see p. 45), has also made out a circulation 

 of the contents of latieiferous vessels. This movement may 

 be generally observed by placing a leaf of the common Ce- 

 landine {CheJielonium me/j'us), previously dipped in oil, under 

 the microscope, and is described by Balfour " to resemble in 

 many respects the appearance presented by the circulation in 

 the web of a frog's foot." AVe have, however, never succeeded 

 in observing such an evident circulation in any latieiferous tis- 

 sues examined by us, although we agree with Schultz, Balfour, 

 Carpenter, and others, that a kind of vital movement of the 

 latex does occur in the uninjured plant. Amici, Treviranus, 

 Mohl, Henfrey, and others, altogether deny the existence of any 

 such a movement in uninjxu'ed tissues, and describe the circula- 

 tion as depending "upon a disturbance of the equilibrium by 

 external causes, such as pressure and heat, and may be pro- 

 duced at will in any direction by making an incision, towards 



