LATICIFEROUS TISSUE. 53 



so that there is a free communication throughout the whole sys- 

 tem. The articulations which they often present (as in the upper 

 part of Fig. 50) seem to prove that they are formed by the con- 

 fluence of cylindrical cells. It appears altogether most probable, 

 however, that the true view of these vessels is that maintained by 

 Meyen, Mohl, Schleiden, and Henfrey ; namely, that they are nei- 

 ther proper tissue, nor composed of cells at all ; but are 'mere pas- 

 sages formed in the intercellular spaces, and which in time ac- 

 quire a proper membrane by deposition from the contained fluid. 

 In this respect, therefore, these vessels may be justly compared 

 with the veins of animals ; but the circulation which Schultz, the 

 discoverer of this tissue, so elaborately described, has been shown 

 to have no existence. There is merely a mechanical flow from a 

 part subject to pressure, or towards a place where the latex is 

 escaping. These vessels are found in the bark, especially in 

 the liber, in the leafstalks, and in the leaves. They are most 

 numerous or conspicuous in those plants in which the fluid they 

 contain becomes white or colored, that is, in those which have a 

 milky juice. 



64. AH the different kinds of tissue that enter into the composi- 

 tion of the plant have now been described, and referred to the cell 

 as their original. Every plant or each organ consists at first of 

 one or more cells of proper cellular tissue ; each doubtless com- 

 mencing with a single specialized cell. In many of the simpler 

 vegetables, the cells multiply in this primitive form solely ; and 

 the fully developed plant consists of parenchyma alone. But in all 

 plants of the higher grades, some of them early assume the forms 

 or undergo the transformations by which they give rise to woody 

 tissue, ducts, or vessels. All these various sorts of modified cells 

 lie vertically in, or conspire to form bundles or cords that run 

 lengthwise through, the stem or other organ they occur in ; so that 

 they may be collectively called the Vertical System or Longitudinal 

 System (56). They accompany each other, and together make up 

 the woody parts, as in the wood proper, in the liber or inner bark, 

 and in the fibrous framework of the leaves. And, while the various 

 kinds run into each other through every manner of intermediate 

 forms (as in the wood of the Yew, for instance, 54), the whole, 

 taken together, compose tissues which are almost always manifest- 

 ly different from the parenchyma in which they are imbedded. It 

 is convenient, therefore, to give to these the collective name of 

 5* 



