of Schizaea malaccana. 507 
stem-apex in Ferns is very scanty, but it appears from the^general 
statements of Van Tieghem that the first-formed tangential walls 
generally mark the external limit of the stele or ring of steles, 
the sheath-layers (pericycle and endodermis) usually arising 
in common with the cortex outside these early tangential 
walls. On this ground Russow 1 , believing that histogenesis 
should be used as a basis for the morphological classification 
of tissues, proposed to exclude these sheath-layers from the 
vascular system. Strasburger 2 , as is well known, proposed 
the name ‘ phloeoterma ’ for the innermost layer of the cortex, 
using this last term for the belt of tissue external to, and clearly 
separate from, the young stelar system during histogenesis. In 
this sense the pericycle and endodermis of monostelic fern- 
stems nearly always arise from the phloeoterma, and it might 
be supposed that this layer had a widespread morphological 
value. But the present case, even should it prove an isolated 
one 3 , in which the sheath-layers arise, with the vascular ring, 
from a single initial layer, would appear to destroy this sup- 
posed morphological value, for we can hardly imagine that the 
sheath-layers are not homologous (that is phylogenetically 
identical) throughout the monostelic Ferns. This is a particu- 
larly striking instance of the untrustworthiness of histogenetic 
differences as guides to the morphological correspondence of 
different regions, a conclusion which can be reached on various 
grounds 4 . 
It is now, we believe, generally accepted by those who con- 
cern themselves with the phylogeny of tissues, that single 
criteria cannot be employed, the only sound method of 
procedure being a careful comparative consideration of the 
structures as a whole from every point of view, the adult struc- 
1 Vergleichende Untersuchungen, 1872, p. 195. 
2 Ban und Verrichtungen der Leitungsbahnen. Hist. Beitr., IV. p. 310. 
3 From appearances observed by Mr. Boodle in transverse sections of the stem 
of Schizaea , it is probable that the type of histogenesis described may be found in 
other species of the genus. 
4 We cannot enter here into the intricacies of the actual relation of histogenesis 
to the morphology of the adult tissues, but it is proposed to publish shortly 
a detailed historical and critical account of the whole subject. 
