igig] BAILEY— BARS OF SANIO 461 



portions of the growth rings of the older secondary xylem. The 

 particular t>'pes of unconformity and bandlike thickenings of the 

 middle lamella which occur in a given species vary considerably in 

 plants grown under different environmental influences and in 

 different organs or regions of a single plant. This is as true ot the 

 first formed as the older secondary xylem. 



Discussion 



It is evident that bandlike thickenings'' of the middle lamella, 

 separating more or less elongated primary pit areas, are not confined 

 to the tracheids of certain Coniferae, but are widely distributed 

 among the Pteridophyta, G>Tnnospermae, and Angiospermae.s 

 Any interpretation of the so-called rims or bars of Sanio in the 

 Goniferae, therefore, should be in general harmony with the 

 structure and distribution of these bandlike thickenings in other 

 groups of vascular plants. 



Since bandlike thickenings of the middle lamella and transitions 

 between scalariform and "alternate" and "opposite" multiseriate 

 pitting are of common occurrence in the younger xylem of many 

 paleozoic and mesozoic (as well as less primitive) plants, the occur- 

 rence of opposite (as well as alternate) pitting and more or less 

 rudimentary Qiierleisten in the transitional tracheids of the cone 

 axes of Araucarians does not indicate conclusively that the Arau- 

 carieae are descended from the Abieteae. Similarly, the more or 

 less sporadic occurrence of alternate pitting, as well as opposite 

 pitting and "bars of Sanio," does not indicate necessarily that the 

 secondary xylem of the Ginkgoales, Abieteae, Taxodieae, Cupres- 

 seae, and Taxaceae is a modification of that which occurs in the 

 Araucarieae or Cordaitales. 



Furthermore, there are a number of facts in the comparative 

 anatomy and ecology of the Pteridophyta, Gymnospermae, and 

 Angiospermae which suggest that unconformity between the pits 

 in the primary and secondary walls of tracheids and vessels may be 



" WMch stain dark blue in sections treated with Haidenhain's iron-haematox^din. 



5 The bandlike thickenings are usually inconspicuous in surface views of the facets 

 of tracheary elements owing to the fact that they are concealed by the thick, super- 

 imposed secondary walls. 



