402 McNicol. — On Cavity Parenchyma and Tyloses in Ferns . 
Matonia , p. 183, but no details are given. Farmer in the same year (’ 99 ), 
referring to Helminthostachys , writes that the protoxylem groups ‘ are often 
seen to border on a band of peculiar parenchyma similar to that which 
Russow termed Luckenparenchym. The cells of this tissue are large and 
thin-walled . . . the general impression which the observer acquires is that 
he is dealing with some kind of gland ’. — p. 439. A somewhat unusual case 
of cavity parenchyma occurs in Loxsoma ) where the cells become pitted and 
lignified 5 as recorded by Gwynne- Vaughan (’ 03 ), who states its formation to 
be due to a kind of tylosis. His figure showing some of the cavity paren- 
chyma cells is reproduced in PI. XXV, Fig. 17. Thus in all the large orders 
of modern Filicales and in Marsilia cavity parenchyma has been mentioned 
as present by different authors, and in one or two cases has been described, 
notably in Loxsoma by Gwynne-Vaughan, and in Gleichenia and Tricho- 
manes Prieurii by Boodle. In the following account of cavity parenchyma 
I shall refer first to a number of ferns, giving details in each case, and shall 
then consider the function of the tissue. 
A typical case of cavity parenchyma occurs in Microlepia platyphylla , 
one of the Polypodiaceae. In the 
petiole of this fern there is a single 
curved bundle, the ends being 
slightly hooked inwards ; there is 
a group of protoxylem near each 
hook, and three or four groups 
lying between them (Text-fig. 1). 
The position of the parenchyma 
strands is shown by crosses. 
An enlarged view of one of the 
median strands in transverse section 
is shown in Pig. 1 of the plate. The cavity parenchyma is well developed, 
though it does not attain the great development of the corresponding tissue 
in the Tree Ferns, which are exceptional in this respect. The following 
description of the method of formation of cavity parenchyma can be 
applied with slight modifications to all Ferns. The tissue is most 
apparent in the thicker parts of the petiole near the base ; it is continued 
up the rachis to the younger parts, gradually becoming less apparent 
towards the apex. Towards the end of the rachis the first stages of the 
development of the cavity parenchyma can be best seen. The xylem- 
sheath cells in contact with the protoxylem are seen to be somewhat 
enlarged : this enlargement results in their being pressed into close contact 
with the spiral vessels of the protoxylem ; with further enlargement the 
parenchyma cells begin to press in between the spirally thickened portions 
of the vessels. The result is that a single parenchyma cell of the xylem- 
sheath generally shows several outgrowths, the constrictions representing 
Text-fig. i. Diagram of the petiolar bundle of 
Microlepia platyphylla. 
E, endodermis. X, xylem. 
PH, phloem. C, wood-parenchyma. 
