STRUCTURAL BOTANY. 31 



The cell-walls are readily permeated by fluids, which 

 pass in and ont through them incessantly ; hence we must 

 regard them as porous. 



73. Yaeieties of Cells are : a^ the Wood-cells — that is, 

 elongated tubular cells or fibres with thickened walls, and 

 grouped in bundles, with their tapering ends overlappino- 

 each other (very fine, long, and tough in the lark^ where 

 they are called hast-cells) ; 5, the DucU^ more or less elon- 

 gated tTibes, either single or combined (they are combined, 

 when formed of a row of cells placed end to end), larger 

 than the wood-cells, and only in rare cases visible to the 

 naked eye. There arc different sorts of Ducts — namely, 

 Dotted Ducts, the dots of which are not holes, but merely 

 thin places in the cell-wall; Spiral Ducts, also called 

 spiral vessels, in which the secondary layers consist of 

 spiral, or ring-shaped fibres or bands, thickening the wall ; 

 finally, there are many other forms met with here and 

 there. 



73. A UNION OF SEVERAL CELLS, forming a coherent 

 mass, is called Cellular Tissue. 



Owing to the various forms and arrangements of the 

 cells, this tissue bears different names — namely : 



a. Parenchyma — that is, ordinary cellular tissue, a sys- 

 tem of rounded, lobed, or stellate cells, with frequent in- 

 terstices ; or of angular, prismatic, polyhedral cells, with 

 but few, if any, intercellular spaces ; 



h. Pleurenchyma — that is, fibrous tissue formed of 

 wood-fibres; and, 



c. Trachenchyma, a tissue consisting of ducts. 



(What is called Cienchyma is nothing but a system of 

 canals and cavities between the cells. Only very rarely it 

 happens that the cell-walls of a tissue come into actual 

 contact. Between most of them there are intervening 



