324 BOTANICAL GAZETTE [april 



It should be noted that the only one of the four species which con- 

 tains starch in the "prismatic layer" is the only one which shows well- 

 marked zonation, and is also the one in which the parenchyma cells are 

 the largest and contain the most protoplasm. 



Discussion of secondary thickening 



A study of the structure of the cells composing the "prismati 

 layer" is of interest only as affording a basis for an interpretation of 

 the nature of the layer. The interpretation which is accepted in 

 the most comprehensive of all recent works on pteridophytes, Bower s 

 Origin of a land flora, as well as in Campbell's Mosses and fern 

 and other current texts, is Russow's theory, which more recently ha- 

 received the indorsement of Scott. As was mentioned above, they 

 look upon the "prismatic layer" as a complex of tissues, consisting of 

 parenchyma, phloem, and xylem. Among recent writers Smith : 

 the only one who has even suggested that the tissue may be of a le 

 extraordinary nature. 



The position of this layer would naturally lead to the conclusion 

 that it is secondary xylem, but for the fact that its composition is not 

 what we have been accustomed to look upon as characteristic of t a 

 tissue. The parenchyma is more abundant, while well-defined 

 tracheids are not only few in number in most species but exceeding} 

 rare or entirely absent in others. In addition to the parenchyma an 

 tracheids, there are the pitted cells, which have been regar 

 phloem. It may be well at this point to consider the evidence upon 

 which this claim is made. Scott and Hill say: 



The phloem elements have an extremely characteristic structure o ^^ 

 walls which comes out conspicuously in sections stained in hematoxy . in ' fonning 

 walls are much pitted, the thicker bands of membranes between the pits ^^ 

 a lattice-like reticulum. The pits are often subdivided by fine bars in o^ ^ 

 areas. Little of the nature of formed contents can usually be detected, J 

 times small, deeply staining globules are found adhering to the walls, an^ ^ 

 ently localized at the pits. In the older parts of the stem the phloem » ^ ^ 

 extent obliterated, dense masses of callus-like substances apF ann 8 ^"^nine- 

 walls and almost filling the cavity. The masses stain like callus wit ^^ 

 soda, but the other callus reactions tried did not give wholly sat,s ° t ' hllS na ve 

 We have not investigated the more minute histology of the phloem an ^ ^ ( 

 not demonstrated the perforations of the thin-walled areas. That ma> ^^ 

 other investigators, but in the meantime, we can scarcely doubt that t e 



ded 



