XV] 



LEPIDODENDRON 



169 



secretory zone are preserved, consisting of large sacs or spaces 

 and small dark cells as seen in figs. 181, B, E, sc, F; 183, 184. 

 This tissue has the same structure as in L. vasculare and in 

 L. faliginosum: it is a striking fact that there are no indications 

 of any additions to the secretory zone even in stems with such 

 a large amount of secondary xylem as in the Dalmeny specimen 

 (fig. 182, a?). If the secretory zone were of the nature of phloem 

 we should expect to see signs of additions made to it in the 



S. 



mMM^ 





5 



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Fio. 184. Lepidodendron WUiischianum. Longitudinal section of the specimen 

 represented in transverse section in fig. 183. 



course of growth. In this connexion it is worth mentioning 

 that in the recent fern Botrychium (Ophioglossaceae) secondary 

 xylem is formed in the stem, but apparently no additions are 

 made to the phloem. The structure of the secretory zone tissue 

 as seen in the longitudinal section fig. 184, S, is also a serious 

 difficulty in the way of accepting the designation phloem as 



