I2O 



ANCIENT PLANTS 



just like the primary bundles in Lyginodendron. Out- 

 side the primary mass of wood is a zone of secondary 

 wood, but the quantity is not large in proportion to it 

 (see fig. 81), as is common in Lyginodendron. 



Though the primary mass is so fernlike in appearance 

 the larger tracheids show series of bordered pits, as do 

 most of the tracheids of the Pteridosperms, in which 

 they show a Gymnosperm-like character. 



B 



W 



P 



Fig. 81. Heterangium 



A, Half of the stele of a stem, showing the central mass of wood s mixed with 

 parenchyma/. The protoxylem groups /. x. lie towards the outside of the stele. Sur- 

 rounding it is the narrow zone of small-celled secondary wood w. B, A few of the 

 wood cells in longitudinal view: p.x., Protoxylem; p, parenchyma. S, Large vessels 

 with rows of bordered pits. 



The foliage of Heterangium was fernlike, with much- 

 divided leaves similar to those of Lyginodendron. We 

 have reason to suspect, though actual proof is wanting 

 as yet, that small Gymnosperm-like seeds were borne 

 directly on these leaves. 



. Medullosa has been mentioned already (see p. 72) 

 because of the interesting and unusually complex type 

 of its vascular anatomy. Each individual stele of the 

 group of three in the stem, however, is essentially 

 similar to the stele of a Heteraneium. 



