326 BOTANICAL GAZETTE [april 



tests applied to the " prismatic layer" do not give any positive results. 

 In the case of the cells in question it can be shown that not only are they 

 not phloem but that they are xylem. 



A careful examination of the "prismatic layer" of such forms a 

 /. Nuttallii and /. melanopoda will reveal the fact that, while the 

 secondary tracheids are far removed in appearance and stainin 

 reactions from the pitted cells, an almost perfect transition seril 



exists between the two types. 



h 



It is possible to trace a series from the tracheids with lignified spiral or 

 annular thickenings, through those with less regular thickenings and 

 with a smaller amount of lignin, to those in which the thickening is 

 very irregular and which have no trace of lignin. A combination of 

 safranin and anilin blue was found to be particularly valuable in 

 revealing slight amounts of lignin. With this variation in w 



lisriificat 



rail 

 the 



fiord much of an 



amount of cell contents. The existence of the transitional stage- 

 leads almost inevitably to the conclusion that the various types of cdls 

 of the " prismatic layer " differ essentially only in their stage of develop- 

 ment, and that the layer accordingly consists of mature tracheids 

 immature tracheids, and parenchyma. While the series in /. echwo- 

 spora var. Flettii is less perfect, it is very suggesti 

 Tuckermani var. Harveyi is usually too limited to i 

 indication of the nature of the pitted cells. 



The recognized steps in tracheid development are as follows: t e 

 more or less regular thickening of the wall; the loss of cell contend 

 and the lignification of the wall. It should not be assumed, because 

 a parenchyma cell undergoes the first steps of the changes whic 

 would lead to the formation of a tracheid, that there is any inheren 

 necessity for their continuance. It is perfectly possible that « * 

 course of development might be arrested at any point, and that any 

 or all of these changes might be incomplete, according to the us 

 standards of completeness. Unfortunately,' the cases of secon WJ 

 growth in modern pteridophytes are so few in number and so hm» 



; " - nities for comparison: wi 



-has 



ther 



very 



this very thing— the incomplete development of the trachea- 

 been described as characteristic of the secondary xylem ot o 

 pteridophytes, and indeed is almost made the test of secondary *J 



