420 Boodle and Hiley. — On the Vascular Structure of 
upper side. The leaves are of the so-called dichotomous form , 1 and have 
long pinnules ; the last feature being one of the characters of the subgenus 
Mertensia , in which this species is placed. 
Internode. 
A transverse section of a fully developed internode is represented 
diagrammatically in Plate XXIX, Fig. i. The cortex (c.) consists of 
brown sclerenchyma, and is bounded externally by an epidermis, and 
internally by a well-marked endodermis. A zone of cortex, two or three 
cells in thickness, and just external to the endodermis, is distinguished from 
the rest of the cortex by the greater thickening of the cell-walls . 2 Three 
layers of thin-walled parenchymatous cells constitute the pericycle, within 
which there is a continuous ring of protophloem with metaphloem on its 
inner side. There is, as a rule, only one layer of parenchyma (mesocycle) 
between the phloem and xylem. The xylem, which has an annular form, 
is mesarch, and is composed of tracheae 3 and parenchyma, intermingled in 
the same way as in other species of Gleichenia . 4 The tissues within the 
ring of xylem form a series arranged in reversed order as compared with 
the similar tissues outside, and are as follows : inner mesocycle, metaphloem, 
protophloem, pericycle, endodermis, and lastly sclerenchyma. The scleren- 
chyma, which occupies the central space, is of the same nature as that 
forming the cortex, and its outer elements usually have smaller lumina than 
those nearer the centre, but there is nowell-marked zone of thick-walled 
cells like that of the inner portion of the cortex. 
The rhizome is distinctly dorsiventral in structure, even where un- 
affected by the proximity of a leaf-trace, since the protoxylems of the lower 
half of a transverse section are differently situated (as regards immersion in 
the metaxylem) from those of the upper half. In the stem, a node of 
which is about to be described, there were seven of the lower kind of proto- 
xylems and four of the upper, but these numbers are not constant, though 
their proportion is approximately maintained. The lower protoxylems 
(which are connected with the roots) are distinguished by having no large 
tracheae outside them ; they are situated in xylem-promontories, in which 
they are covered by only a few small scalariform tracheae. On the other 
hand, the protoxylems of the upper part of the xylem appear deeply 
embedded, as the tracheae on the outside are large, like those on the inner 
1 See Goebel (’05), vol. ii, p. 319 ; Tansley (’07), p. 135 ; Bower (’08), p. 553. 
2 There is sometimes one layer of thin-walled cells between the endodermis and this zone of 
specially thickened cells. 
3 These are to be classed as vessels. Some of the mature tracheae were carefully examined, 
and the pit-membrane was found to be missing. See Gwynne- Vaughan (’08). 
4 The parenchymatous elements of the xylem may become lignified; one case was noted in 
which nearly all the xylem-parenchyma had undergone this change. 
