Bancroft\—On the Xylem Elements of the Pteridophyta. 751 
3. Embedding.—For microtoming, small pieces of material from which 
all hard cortical layers had been removed were dehydrated, cleared in 
cedar-wood oil, and embedded in paraffin (M. P. 49°-5i°). 
4. Sectioning.—The microtome-sections used were mostly 6 or 8 //, in 
thickness. In the majority of cases both longitudinal and transverse sections 
were cut. It was recognized that a comparison of the two was necessary for 
the satisfactory demonstration of the middle lamella, and also for the 
explanation of the presence or absence of spaces between contiguous xylem 
elements. As Halft has pointed out, it is difficult to obtain good longitudinal 
sections with the microtome, owing to the hardness and structure of the 
material, and to the nature of the embedding medium. Free-hand sections 
were found, in the case of the larger and harder specimens, to give more 
satisfactory results. 
5. Staining.—A combination of ruthenium red and methylene blue 
was most generally employed. P'or microtome-sections, the ruthenium 
red solution was of the strength mentioned by Gwynne-Vaughan ( 2 , p. 519). 
For hand-sections, the stain was used very much stronger, only small 
quantities of solution being made at a time, since the fluid stain soon loses 
its power. In this way much better results were obtained, the pectic 
{ middle substance’ staining a bright red. 
Halft recommends Haidenhain’s iron-haematoxylin and haematoxylin- 
alum, used singly. These stain the middle lamella, but do not affect, or at 
least only slightly, the lignified bars. In the present investigations both 
these stains were used with good results. 1 
Other stains found to be effective were Bismarck brown and methyl 
green ; aniline blue and carthamine ; Delafield’s haematoxylin and safranin ; 
the first named in each case differentiating the primary substances. 
A microscopical examination of sections, prepared as described, affords 
conclusive evidence as to the accuracy of the inferences drawn from micro¬ 
chemical and physical tests respectively, and demonstrates that the xylem 
of Pteridophyta is typically composed of tracheides, the middle lamella 
and pit-membranes remaining in both end and side walls. 
C. Discussion. 
Comparing the views of the two most recent writers on the subject of 
the Pteridophyte xylem elements, it will be seen how entirely conflicting 
they are :— 
Halft holds that the pit-membranes remain ; Gwynne-Vaughan, that 
they disappear, the pits being actual perforations. 
1 Zimmermann (Botanical Microtechnique, 1893, p. 142) notes that haematoxylin is most 
satisfactory for the recognition of the closing membrane of bordered pits. He used principally 
Bohmer’s solution. 
3 D a 
