STRUCTURE OF FOSSIL PLANTS 



61 



W 



been remarked already that increase in complexity of 

 the plant form usually goes with an increase in com- 

 plexity of the cells and variety of the tissues. The 

 general ground tissue in nearly all plants is very similar; 

 it is principally in the vascular system that the advance 

 and variety lie. 



Plant anatomists lay particular stress on the vascular 

 system, which, in comparison with animal anatomy, holds 

 an even more important position than does the skeleton. 

 To understand the essen- 

 tial characters of stems, 

 both living and fossil, and 

 to appreciate their points 

 of likeness or difference, 

 it is necessary to have 

 some knowledge of the 

 general facts of anatomy ; 

 hence the main points on 

 which stress is laid will 

 be given now in brief 

 outline. 



Leaving aside con- 

 sideration of the more 

 rudimentary and less de- 

 fined structure of the algae 

 and mosses, all plants 

 may be said to possess a " vascular system". This is 

 typically composed of elongated wood (or xylem) with 

 accessory cells (see p. 57, table), and bast (phloem), also 

 with accessory cells. These specialized conducting ele- 

 ments lie in the ground tissue, and in nearly all cases are 

 cut off from direct contact with it by a definite sheath, 

 called the endodermis (see p. 55, fig. 26). Very often 

 there are also groups or rings of hard thick-walled cells 

 associated with the vascular tissues, which protect them 

 and play an important part in the consolidation of the 

 whole stem. 



The simplest, and probably evolutionally the most 



Fig. 36. Diagram of Simplest Arrangement 

 of Complete Stele in a Stem 



w, Central solid wood ; P, ring of bast ; 

 E, enclosing sheath of endodermis ; C, ground 

 tissue or cortex. 



