66 



ON THE PLANT-CELL. 



longest. The branched liber-cells (fig. 63.), on account of their branch- 

 ing, are included in the following forms. 



Upon the origin of the liber -cells of Apocynacece and Asclepiadacece no 

 observations have been made : only thus much is certain, that they fre- 

 quently contain milky juice. They are found singly, or in little bundles, 

 near to, or in the place of, the liber-bundles ; and are sometimes branched, 

 ex. gr., in Hoya carnosa (according to Meyen), and very beautiful in Sar- 

 costemma viminale. The configuration of their walls is entirely the same 

 as in old milk-vessels. 



The Milk-vessels, in relation to their origin, have been at present but 

 little examined. They appear at first as enlarged intercellular passages, 



64 



65 



and without any visible membrane (fig. 65.) to form them. Nor does any 

 membrane appear to exist over the furrow formed by two neighbouring 

 cells, as it does in all true cells, In old vessels, also, we often find impres- 

 sions and projecting angles, showing that they must have fitted accurately 

 into the surrounding cells (66. A, B). They are mostly branched in so 

 compound a manner, that it is not often possible to examine a cell in its 

 entire length (fig. 67.), yet it is easy to separate it into its parts if the tissue 

 is treated with nitric acid. "Without this means it is easily seen that they 



64 Intermediate formation between liber-cells and milk-vessels from the bark of Cero- 

 pegia dichotoma. The spiral stripes are drawn only in one half. 



65 Milk-vessels from the leaves of Limnocharis Humboldti. The walls of the upper 

 part of the vessel at a are fallen together. The arrows show the direction of the cur- 

 rents. Every milk-vessel is enclosed by two rows of smaller and longer cells of 

 parenchyma. 



