COMPARATIVE ANATOMY OF GYMNOSPERMS 299 



to that which constitutes the external portion of the funda- 

 mental tissue of the stem. 



Fig. 109, /, shows a central cylinder of Osmunda cinna- 

 momea, where not only an internal endodermis is present but 

 also internal phloem as well. In Fig. 109, J, a part of the 

 wall of the same central cylinder is shown more hiffhlv mac- 

 nified. The sieve-tubes are easily recognized as large, appar- 

 ently empty elements. It has been suggested by Jeffrey 10 and 

 Faull, 18 as a result of the study of the anatomy of the whole 

 order, that the type of central cylinder found in the Osmunda- 

 ceae is the result of reduction from a siphonostelic condition 

 with internal phloem. This view of the matter is strengthened 

 by the fact that brown sclerenchyma is sometimes found in the 

 pith of Osmunda regalis and Todea barbara, although in these 

 species there is no longer any communication between pith and 

 cortex in the region of forking. Moreover, exactly similar 

 series of degeneration to that supplied by the Osmundaceae 

 have been shown to exist in the case of certain polypodiaceous 

 ferns. Hence it may be assumed, in the present connection, 

 that the type of central cylinder exemplified by the Osmunda- 

 ceae has arisen by degeneracy from the siphonostelic type with 

 internal phloem ; and that the medulla often shows signs of its 

 origin by striking histological resemblance to the cortex, even 

 when there is no longer any communication between the med- 

 ullary and cortical fundamental tissues. 



Fig. 109, K, shows the structure of one of the tracheary 

 strands of Osmunda cinnamomea. The protoxylem or primi- 

 tive wood appears as a cluster of small elements, just external 

 to a mass of wood-parenchyma. The protoxylem does not abut 

 immediately on the pith, as in the seed-plants, but is separated 

 from it by a considerable amount of wood-parenchyma and me- 

 taxylem ; most of the metaxylem, however, lies external to the 

 protoxylem. This type of tracheary bundle is very character- 

 istic of the ferns, and has been designated " mesarch." 



In the case of the Lyeopodiales, the tracheary bundle is of 

 still another type. If Fig. 108, E, be examined, it will be seen 

 that on the left of the central cylinder, opposite the leaf-trace 

 in the cortex, is a cluster of protoxylem. The primitive wood 

 in this case is external and next the phloem. This feature is 

 very characteristic of the Lycopods and their allies. Bundles 



