40 
A. C. Seivard. 
are at present concerned, but such traces as exist favour the view 
expressed at greater length elsewhere, that the method of secondary 
thickening in Lepidodendroid plants differed considerabiy from that 
with which we are familar in recent plants, or in such fossil genera as 
Catamites , Lyginodendvon , Splienopliyllum and others.” 
“ Beyond the meristematic region there is a characteristic band 
referred to as the secretory zone. This tissue forms a well-marked 
feature in stems of Lepidophloios fuliginosus, L. Wunschianus, and 
other species ; the elements composing it do not conform to the 
structural characters of ordinary phloem. The large clear spaces or 
sacs which form so prominent a feature in this region cannot, I believe, 
be satisfactorily explained as the result of decay previous to mineral¬ 
isation ; their appearance is strongly suggestive of sacs or spaces, 
formed, for the most part, during the life of the plant, by the 
separation and partial disorganisation of thin-walled cells. The 
constant occurrence of patches of a dark brown substance in this 
zone also points to the secretory nature of the tissue . . . The 
group of secretory tissue accompanying the xylem of a leaf-trace 
may be followed, in longitudinal and in transverse sections, into 
direct continuity with the broad secretory zone of the stem; but the 
elements of this tissue which form part of a leaf-trace, differ from 
the secretory zone in the narrower diameter of the cells and in the 
somewhat greater abundance of secreted substance. Neither in the 
structure of the main stele of the stem nor in the tissues of the leaf- 
trace do we find any set of elements, which exhibit histological 
features affording satisfactory evidence of the existence of either 
hard or soft phloem of the ordinary type .... The apparent 
absence of any well-defined phloem tissue is a fact of considerable 
interest; even in the large Dalmeny stem, in which the secondary 
xylem is 2 f 8 cm. in diameter, there is no indication of any tissue 
which can be identified anatomically with true phloem. Indeed in 
no stem of Lepidodendron, Lepidophloios or Sigillaria has typical 
phloem been so far satisfactorily demonstrated. The existence of 
the secretory zone suggests a physiological comparison of this tissue 
with normal phloem, on the ground that in certain recent plants 
laticiferous tissue has been considered as in part at least carrying 
out the functions of phloem; it is a probable view that this well- 
marked zone may have served the same purpose as the tissue which 
in recent plants usually presents the structural characters of 
phloem . . . Without discussing the question at greater length, 
the conclusion may be briefly stated, that the functions usually 
