WOODY TISSUE. 25 



not unfrequently grouped into little tufts. They 

 may be readily found in a piece of the stem of 

 garden-rhubarb (PL I. fig. 5 a), or of the common 

 balsam. 



Porous and spiral cells. The walls of the cells of 

 cellular tissue are sometimes covered with little 

 dots (PL I. fig. 11 a), or slit-like markings ; the cells 

 are then called porous cells. A specimen of them 

 may be obtained from a section of the pith of the 

 elder (Sambucus nigra). 



Sometimes cells exhibit the appearance of a spiral 

 line marking their walls, as if a little bell-spring were 

 coiled up in them (PL III. fig. 2 a). These are called 

 spiral cells, or spiral fibrous cells, and the tissue 

 formed by them is called fibro-cellular tissue. 



We now leave the cells of ordinary cellular tissue, 

 to examine those in which the dimension of length 

 predominates, so that they form tubular cells ; and 

 first of those required to possess strength and firm- 

 ness, combined with flexibility. These qualities are 

 met with in the cells constituting 



Woody tissue. Of this there are two forms, called 

 respectively wood-cells and woody fibres. 



The wood-cells are moderately long, more or less 

 tapering and overlapping at the ends ; and the cell- 

 walls are thickened, so as to possess considerable 

 firmness. These cells are found in the wood of stems, 

 as in the white woody portion of an ash stick, that of 

 a lime-tree, the stem of a Chrysanthemum, &c. (PL I. 

 fig. 6) . They are closely packed, and the tissue formed 

 by their union is called prosenchyma (TT/OO?, close, 

 ey^y^a, tissue). 



In the other kind of woody tissue the cells are 

 very long and slender, strong, yet flexible, gradually 

 tapering at the ends, where they overlap each other ; 

 and they have thick walls, so that, when divided trans- 

 versely, the cavity appears almost filled up (PL I. 

 figs. 5 d, 9, & 7 b) . This tissue is called woody fibre 



