284 BOTANICAL GAZETTE [october 



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medullation (figs. 22, 23). In the change from the protostele to a 



siphonostele there is a rapid increase in the diameter of the stem, 



and an appearance of parenchyma cells with included resinous 



storage similar to those of the pericycle. These increase rapidly 



in number with the appearance of a permanent medulla, but so far 



as observed no real sclerenchyma of the type found in the adult 



stele occurred in young stems up to the time of the separation of 



the fourteenth leaf trace (fig. 24). This early organization of the 



medulla takes place without the appearance of internal phloem, | 



nor could sieve cells be demonstrated at any level in the central 



parenchymatous elements of the young stele. It was further 



observed that there was no endodermal invagination accompanying 



medullation, or the departure of traces from the young stele, up 



to that time. True leaf gaps occur in the stele after the appearance 



of the medulla, and the traces take their departure in a manner 



already described by Sinnot (ii). 



The storage parenchyma cells already mentioned as being very I 



prominent in the early medulla have been observed to occur as 

 early as the time of separation of the third trace, but more com- 

 monly about the time the sixth trace leaves the central cylinder. 

 The first elements of this character are usually located opposite 

 the place of exit of a trace and appear free in the sheath paren- 

 chyma (fig. 2), or occur at the edge of a root attachment. In a 

 few instances they have proved to be continuous with like elements 

 of the pericycle layer, the storage contents of which cells they re- 

 semble. The most pronounced intrusion of such cells observed in 

 any of the young stems is shown in fig. 44, where they occur oppo- 

 site a trace and at the edge of a root attachment. In no instance, 

 however, has it been possible to demonstrate any thickenings on ■ 



the walls of these elements such as occur on the endodermis, I 



and it seems that their relation to the pericycle is not at all constant. 



The endodermis is organized early in the development of the 

 sporophyte, and is continuous over the early stem, the primary 

 root, and the tissues which elongate and diverge into the foot. At a 

 comparatively early stage it closes over the conductive elements in 

 the foot, and thus entirely incloses the stele from the root apices 

 to the meristematic region of the stem apex. A transverse section 



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