Boodle . — Anatomy of the Schizaeaceae . 401 
embedded in the xylem, and these no doubt are derived 
from xylem-parenchyma ; some are found just outside the 
xylem, and a few of these are in contact with the phloem, and 
may perhaps represent sieve-tubes ; and again among the 
large central mass of sclerotic elements, a few of the fibres 
adjacent to the incurved ends of the phloem-zone may be 
lignified sieve-tubes, but the majority of the internal fibres 
are larger elements than the sieve-tubes, and in the median 
upper region they correspond closely in size with the adjacent 
parenchyma, which lies between them and the endodermis. 
Gwynne-Vaughan (’ 01 , p. 86) suggests that perhaps both 
sieve-tubes and parenchyma are implicated in the sclerosis. 
As stated above this may be the case, but, as far as present 
data go, the fibres may on the other hand all be parenchyma- 
tous. In that case they would be of a different nature from 
those of the Schizaeaceae. Outside the Schizaeaceae, 
Hymenophyllaceae, and Gleicheniaceae, fibres in the bundle- 
tissue appear to be rare among Ferns. The example given 
by Thomae (’8f, p. 129) is that of A diantum trapeziforme. 
Fig- 45 is a drawing of a small piece of one of the petiolar 
bundles. The fibres (/.) occur both in the xylem among the 
tracheides, and on the inner and outer side of the xylem band ; 
those at the top right-hand corner of the figure being on the 
inner side. It is no doubt, as Gwynne-Vaughan suggests, 
a case of sclerosis of xylem-parenchyma. Probably few if 
any of the outer fibres represent sieve-tubes, so most at any 
rate of these sclerotic elements belong to a different category 
from the fibres of the Schizaeaceae. Fig. 44, which represents 
half the petiolar bundle of Nephrodium Thelypteris , is included 
here to illustrate the occurrence of thick-walled, unlignified 
elements, apparently sieve-tubes, in a member of the Poly- 
podiaceae. They occur within the hook of the xylem. 
Similar elements are mentioned by Gwynne-Vaughan (’01, 
p. 85) in Davallia , &c. 
