Anatomy of Solenostelic Ferns : I. Loxsoma . 85 
resemble sieve-tubes in their structural details, being very 
elongated and tubular, and arranged more or less in series 
with very oblique terminal walls. The walls are greatly 
thickened and more or less lignified, but even when strongly 
so they are easily distinguished from the tracheides by the 
rounded inner contour of their lumen (Fig. 6). Two fairly 
distinct layers of thickening have been deposited upon the 
middle lamella. A number of round or elliptical simple pits 
are scattered over the walls (Fig. 10 ), and in immature stages 
(especially on the terminal walls) they are so numerous and of 
such a size that they give the impression of a coarse reticula- 
tion, reminding one strongly of the sieve-tubes themselves. 
They never appear to be quite empty, but have much denser 
contents than the sieve-tubes, and nuclei are frequently to be 
observed in them. It would seem, therefore, that both their 
distribution and their structure would most readily be explained 
if they were regarded as elements of the phloem originally 
designed for sieve-tubes, but which, in the course of their 
development, have thickened and lignified their walls to such 
an extent that the areas which should have become sieve- 
plates have become simple pits, and the whole element more 
closely resembles a fibre than a sieve-tube. It is not implied, 
of course, that they were all at one time actually functional 
sieve-tubes. Further support for this point of view may be 
drawn from the fact that a number of elongated elements 
with thick two-layered but unlignified walls, occupying pre- 
cisely the same position in the petiolar meristele as do the 
fibrous elements in Loxsoma , may be found in many Micro- 
lepias ( Davallia platyphylla :), and Dennstaedtias (Dick sonia 
punctiloba ), where they can only be interpreted as repre- 
senting sieve-tubes. 
Although the above explanation seems highly probable 
for the particular plants in question, yet it certainly is not 
safe to apply it to all the other cases in which fibrous elements 
have been found in the petiolar meristeles of Ferns. However, 
it seems to account satisfactorily for the cases of Lygodium , 
Schizaea , and Aneimia mentioned by Prantl (1. c.), and also 
