240 



GYMNOSPERMS 



growth immediately becomes very vigorous, so that the rings arc 

 many times wider than under bog conditions (fig. 247). 



Histology. — The woody cylinder of all the conifers is, typically, 

 an endarch siphonostele; but there are many traces of the ancient 

 mesarch condition which characterized their remote Pteridophyte 

 ancestors. Centripetal wood is more prevalent in the Taxaceae and 



tnt 



amavack StMomo 



tt. 



\*'^ 



Fig. 247. — Tsugaca>i>ut()isr: transvtrsi' sii tion ol' stim. The small rings were grown 

 under bog conditions; the larger ones, after the bog was drained. 



Podocarpaceac than in the other families, so that, in this respect, 

 they are more primitive. It is claimed that Cephalotaxtis has a 

 mesarch cylinder; it is certain that the bundles of the cotyledons arc 

 mesarch. In Taxiis and Torrcya the strand is mesarch at the base of 

 the cotyledons, but changes to exarch higher up. The bundles in the 

 phylloclads of Phyllocladiis are mesarch. Hill and de Fr^mne^^' 

 examined the position of the protoxylem in all the families of the 

 conifers, paying special attention to the "rotation" of the protoxy- 



