328 BOTANICAL GAZETTE [april 



the nature of the pitted cells of Isoetes. In the apices of Lycopodium. 

 where the xylem is not yet mature, it is not difficult to find tracheitis 

 that have round or oval pits, and which before they are lignified pre nt 

 a similar appearance to that of the pitted cells of Isoetes. In fact, in 

 L. pithyoides, the fully developed sieve cells and the pitted tracbefcfe 

 in which lignification has not yet taken place differ only in the greater 

 regularity of the pitting of the tracheids. If the development of th 

 more slender tracheids of Lycopodium were arrested before lignitica- 

 tion had begun, it would be difficult to distinguish sieve cells from 

 tracheids except by position, since both have so nearly the same general 

 appearance and the same reactions to stains. As the disposition ot 

 the thickening of the primary tracheids of Isoetes is much less regular 



mi 



secondary xylem would also show irregularities. 



One of the reasons given by Russow and subsequent observers tor 

 regarding the "prismatic layer" as part phloem, is that it is in direct 

 continuity with the phloem of the leaf traces. I am not disposed to 



oem 



There 





is nothing in its position and structure to cause any hesitation a*w 

 accepting it as phloem. The sieve tubes possess well-defined »£ 

 plates, and the tissue as a whole is so definitely marked off, bot m 

 position and development, from the xylem and all adjacent iss 

 that there is no apparent reason for questioning its identity a* p • 

 There can be no question of the continuity of the old leaf traces a ^ 



m<- jjumiuuic layer in sum cases as die »»«/"— — i •-> ,j 



in the case of young leaf traces the point is not so certain. ^ 



cations are that the continuity is a result of the overgrow t _^ 



leaf traces by the secondary tissues. Fig. 17 is a dia £ ram , 

 :n„_^ .;. ., . . -r ....:.. .ji^qps where sucna 



v,^«*o ... some 



large 



continuity exists. In any old stem there are present a ^ .^ ^ 

 number of leaf traces, of which only a comparatively few a . ^ matic 

 are alive and active. The upper part of the stem is a mens 



A the zone ^ iu/ 



region, composed of the meristematic leaf traces ana iu - ^ ^ ^ e 

 lower projection is the cambium. This region fe . ^i traces <rf 

 diagram by fine dots. The xylem strands of the various ea ^ ^ 

 that level move in together, forming a more compact ns, ^ ^ 

 vascular axis. The phloem ends in the parenchyma, t re 



