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A TEXTBOOK OF THEORETICAL BOTANY 



A 



V 





Fig. 633. — Equisetum arvefise. Vascular structure of the stem. 

 A, Early stage near the apex, internodal protoxylem strands 

 and leaf trace strands still independent. Outline of leaves 

 shown by dotted lines. B, Internodal protoxylem strands 

 and leaf traces united, but internodal strands still independent 

 of those in the internode above. C, Development of 

 metaxylem at the nodes connecting the internodal pro- 

 toxylem strands, which are now surrounded by carinal 

 canals which are transversely shaded ; metaxylem dotted. 

 D, Basipetal extension of the metaxylem in the internodes. 

 Branch traces have been omitted. The nodal branches of 

 the protoxylem strands lie behind the nodal metaxylem 

 and do not appear in the figure. 



B 



D 



