THE ANATOMY OF WOODY PLANTS 



The examples of cells hitherto considered have been those which 

 in their functional condition contain living protoplasm. It is now 

 necessary to refer to those very important histological elements 

 of vascular plants, which in the mature condition normally con- 

 tain no protoplasm and serve to conduct water. Elements of 

 this nature are typically much elongated and are provided with a 

 different sort of intercommunicating pitting than that found in 

 the case of those which in the active state shelter a protoplasmic 

 body. In the accompanying figure (Fig. 4) are seen a number of 

 cells of this sort cemented together by the middle lamella (repre- 

 sented as a black framework around 

 the cell walls). The pits which 

 bring about intercommunication 

 are n S ure d as occurring on two of 

 the four nearly parallel sides of the 

 tracheids or water-conducting cells. 

 Their openings have overhanging 

 margins, and the membrane, the 

 Diagram of simple and presence of which precludes the 



possibility of actual openings, is 

 much thickened in the middle to 

 constitute the so-called torus. The 

 presence of overhanging margins clearly distinguishes this type 

 of pore from that found in the walls of living cells. The cement 

 substance uniting the cells with one another is in general lignified 

 (that is, has undergone that somewhat complex and obscure 

 modification chemically known as lignification) like the cell walls 

 which it holds together, but is likewise partially in the pectic or 

 mucilaginous condition, a state which causes it to absorb hema- 

 toxylin strongly. In the region of the pit membranes the middle 

 lamella becomes pectic cellulose, and here water passes through the 

 walls much more readily than it does elsewhere. On either side 

 of the figure is seen a ray of the wood, the cells of which are in 

 relation with the tracheids by means of pits. It is clear that the 

 pits which bring about intercommunication are bordered on the 

 side of the elongated element of the wood (the tracheid) and are 



FIG. 5. 



bordered pits in face and profile views. 

 In the center a bordered pit from 

 heartwood is shown. 



