Lecture XIX. 149 



contribute cord-like tracts to the net-work of conducting tracts 

 in the stem. There are usually about three roots which emerge 

 from the base of each leaf. Each of these roots has an axial 

 conducting tract which passes from the root into the base of 

 the leaf-stalk just above the junction of the latter with the stem. 

 It then makes its way across the outer fundamental tissue of 

 the stem and joins with a tract of the net-work. The tracts 

 which are given off from the meshes to the bases of the leaves 

 pass up the leaf-stalks into the leaf-shafts and out into the mid- 

 ribs of the pinnae. Finally they pass into the pinnules and there 

 they bifurcate and form the veins in the leaf. Thus there is a 

 conducting system continuous throughout the whole plant from 

 root to leaf. 



The fundamental tissue of the stem is composed of rounded 

 cells with moderately thin walls. Their diameter in different 

 directions are approximately equal. Such a tissue is often described 

 as parenchyma. The cavity of these cells is chiefly occupied 

 with starch grains and mucilage, which have been deposited 

 by the protoplasm of the cells. Between the cells there are 

 angular intercellular spaces which contain air. Each conducting 

 tract has a fine core of woody material which is tough and 

 elastic. It forms what is known as the wood, or xylem, of the 

 strand. Surrounding this is a softer and more juicy substance 

 which is known as the bast or phloem of the tract. These 

 tissues are encased in a thin hard layer, dark brown in colour, 

 which forms a tubular coat enclosing the whole tract. This 

 is the bundle-sheath. 



The greater part of the wood, or xylem, is formed of long 

 narrow tubes tapering at each end. At their widest they are 

 about 0-05 mm. in diameter, and are about 1-5 mm. long. These 

 tubes are called tracheids. In transverse section they appear 

 polygonal. The walls of these tubes are formed of wood which 

 is a substance derived from cellulose, differing from it in its 

 physical and chemical properties. It is more rigid, more perme- 

 able to water and less ductile than cellulose. The walls are of 

 considerable thickness but have thin places upon them called 

 pits. The pits of the tracheids of the ferns usually have the 

 form of transverse .slits which are quite close together, so that 

 the thick portions between them may be compared to the rungs 

 of a ladder. Hence these tubes are called scalariform tracheids. 

 The tapering ends of the tracheids overlap each other in the* 

 xylem core and where they are in contact with one another the 

 pits on the adjacent walls correspond, so that at these points the 



