5 



ANCIENT PLANTS 



outside becomes increasingly great in comparison with 

 those forming the external layer. Some idea of the 

 complexity and differentiation of this cell mass is given 

 in fig. 19, A, which shows the relative sizes and shapes 

 of the cells composing a small part of the stem of a 

 common flowering plant. The complete section would 

 be circular and the groups v would be repeated round 

 it symmetrically, and the whole would be enclosed by 



n 



ss 



ss 



Fig. 20. Conducting Cells and Surrounding Tissue seen in fig. 19, A, cut 

 lengthways, px, First formed vessels for water conduction; .r, larger vessel; 

 b, food-conducting cells; ss, strengthening cells; p, general ground tissue. 



an unbroken layer of the cells marked e, as in the 

 diagram B. 



In the tissues of the higher plants the most impor- 

 tant feature is the complex system of conducting tissues, 

 shown in the young condition in v in fig. 19, A. In 

 them the food and water conducting elements are very 

 much elongated and highly specialized cells, which run 

 between the others much like a system of pipes in the 

 brickwork of a house. These cells are shown cut longi- 

 tudinally in fig. 20, where they are lettered to correspond 

 with the cells in fig. 19, A, with which they should be 



