On Cavity Parenchyma and Tyloses in Ferns. 
BY 
MARY McNICOL, B.Sc., 
Beyer Fellow in the University of Manchester. 
With Plate XXV and seven Figures in the Text. 
T HE term £ cavity parenchyma * has been used to indicate a special kind 
of tissue occurring in the petioles of some Ferns in contact with the 
protoxylem groups. It consists of soft cells of a more or less irregular 
shape, forming vertical strands of loose large-celled parenchyma which 
replaces to a greater or less degree the first-formed elements of the xylem. 
Such parenchyma is formed from the one-layered sheath of soft cells 
immediately surrounding the protoxylem. The sheath cells in contact with 
the spiral vessels become enlarged and send ingrowths between the spiral 
thickenings into the lumina of the vessels. The result of further protrusion 
of the parenchyma cells into the vessels is that more or less spherical, 
swollen processes are formed, and the vessels become gradually broken up 
by the increasing pressure of these processes within the passages, the place 
of the spiral vessels being eventually taken by the parenchyma so formed. 
Russow proposed the name ‘ LUckenparenchym ’ for the soft cells filling up 
the cavity left by the tearing of the tissues near the protoxylem in 
Mars ilia. 
One of the earliest writers to mention strands of soft cells was Dippel 
(’64), who noticed them in Osmunda and in Cyathea microlepis ; Terletzski 
(’84) also found them in Pteris aquilina and Struthiopleris germanica : he 
considered them to be part of the wood tissue, for, he says, they border 
directly on the protoxylem, and never directly on the bast cells and sieve- 
tubes, from which they are always separated by the parenchymatous sheath. 
Boodle (’01) gives a short account of cavity parenchyma in the Schizaeaceae ; 
he states that in Aneimia there are three groups belonging to the conjunctive 
parenchyma adjoining the xylem, one at the median point, the others at 
a short distance from each hook ; in Mohria there are also three groups of 
parenchyma (p. 359)- In the Gleicheniaceae he mentions it as occurring in 
nearly all species, the cells frequently becoming thick-walled and lignified. 
Seward and Ford (’03), writing of Todea , state that £ the parenchymatous 
tissue abutting on the protoxylem strands occasionally forms small irregular 
cavities (cavity parenchyma),’ p. 247 ; Seward (’99) mentions it also in 
[Annals of Botany, Vol. XXII. No. LXXXVII. July, 1908.] 
