310 MORPHOLOGY OF ANGIOSPERMS 



the vascular plants, viz., exarch and centripetal. The root of 

 the Spennatophytes is consequently conservative, and retains 

 intact ancestral pteridophytic features. It seems phylogenet- 

 ically significant that the exarch type of wood, so typical of 

 the Lycopods and their allies, is always present in roots, and 

 never the mesarch type so characteristic of the Fern-alliance. 

 This feature probably indicates that the Lycopod stock is an 

 extremely old one, a conclusion borne out by the fact that the 

 Lycopsid series had already culminated in the Carboniferous 

 age. It appears also not improbable that the Pteropsida, large- 

 leaved fern-like plants, took their origin from the microphvl- 

 lous lycopodinean stock in remote antiquity, and still exhibit 

 a trace of their origin in the primary structure of their roots. 



GXETALES 



This group is generally regarded as the highest of the Gym- 

 nosperms, a view which is borne out both by a consideration 

 of its anatomy and its reproductive organs. The latter show 

 in the case of Tumboa and Gnetum a considerable advance 

 toward the condition of true flowers, and this advance is paral- 

 leled by a reduction in the amount of female prothallial tissue 

 antecedent to fertilization. The Gnetales on the anatomical 

 side show indubitable evidence of gymnospermous relationship, 

 in the presence of quite typical foliar transfusion tissue. They 

 are distinguished anatomically from all other Gynmosperms, 

 however, living or fossil, bv the presence of rudimentary vessel.-. 



Fig. 113, CC, shows the structure of the wood in Gnetum 

 Gnemon. The secondary wood in this species consists of tra- 

 cheids and vessels, the latter being easily distinguished by their 

 larger size. In some cases the fact that direct communication 

 between two contiguous vessels is merely the result of the dis- 

 appearance of the membrane of a bordered pit can be made 

 out.* 



* For list of literature cited see end of Chapter XVII. 



