STRUCTURAL BOTANY. 31 



The cell-walls are readily permeated by fluids, -whicli 

 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 overlapping 

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

 they are called last-cells) ; I, the Ducts, more or less elon- 

 gated tubes, 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 are different sorts of Ducts — namely. 

 Dotted Ducts, the dots of which are not holes, biit 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 Cellulae 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 ; 



I. 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 



