200 



CONIFERALES 



[CH. 



horizontal rows is a characteristic feature. The characters of 

 Taxodioxylon may be summarised as follows: Annual rings 

 distinct ; bordered pits on the radial walls of the tracheids in 1 4 

 rows, circular and separate and, if in two or three rows, opposite ; 

 rims of Sanio present. Medullary-ray cells pitted only on the 

 radial walls ; pits in the field, 2 8 in number, often arranged in 

 horizontal rows (fig. 720, A), sometimes fairly large, simple, or 

 bordered and horizontally elliptical. The thick walls of the xylem- 

 parenchyma (fig. 720, B) are characteristic of Taxodium and 

 Taxodioxylon and this character has been quoted as a trustworthy 







 



A B C 



FIG. 720. A, B, Cupressinoxylon (Taxodioxylon) Taxodii. C, Cupressinoxylon (Taxo- 

 dioxylon) Sequoianum. (A, B, after Gothan; C, after Mercklin.) 



distinction between Taxodium and Sequoia sempervirens though, 

 as already pointed out, this is not a safe test 1 . In Glyptostrobus 

 heterophyllus the transverse walls of the xylem-parenchyma are 

 also thick and pitted and this tissue in some species of Cupressus 

 exhibits precisely similar features. It is in certain inconstant 

 features that Taxodioxylon differs from Cupressinoxylon. In 

 several instances the occurrence of wood referred to Taxodioxylon 

 in beds containing impressions of foliage-shoots like those of 

 Taxodium and Sequoia sempervirens strengthens the conclusions 

 based on anatomical characters. 



1 See page 149. 



