Scott and Hill.—Structure of Isoetes Hystrix. 435 
of the ligule, at the base of the labium. When the leaf is 
mature, this sub-ligular xylem, which is not so strongly 
lignified as that of the foliar bundle, may nearly reach the 
latter, but we have never observed actual continuity between 
the two tissues. 
The leaf-traces, where they pass through the cortex of the 
stem, are collateral in structure. Near the stele, the xylem, 
which is well developed, is made up of elements in close 
contact with one another, while in the outer cortex the 
tracheides are separated by parenchymatous cells. The 
phloem is well marked ; the elements composing it, which 
are separated from the xylem by parenchyma, present all 
the characteristics of the protophloem of the leaf, in shape, 
size, thickness of walls, and the decided blue colour which 
they take up when stained with haematoxylin. 
The leaf-traces, where they traverse the outer cortex of 
the stem, are practically identical in structure with the bundle 
of the leaf in the region of the sporangium. 
Immediately before joining the central wood, the leaf-trace 
of course shows no phloem ; here it is surrounded by a well- 
defined zone of parenchyma, the outer boundary of which 
marks the limit between the leaf-trace and the surrounding, 
growing tissue of the intracambial zone. 
The development of the leaf is well known, and may be 
briefly dealt with. It arises as a dome-shaped mass of 
tissue, from the superficial layers of the apex, and grows 
at first towards the centre of the apical depression. The 
ligule is the first part of the leaf to be differentiated. It 
arises from a single cell, which grows enormously in size, 
while the leaf is still quite small. The first divisions in this 
cell are transverse. Longitudinal growth takes place very 
rapidly, so that the ligule becomes bent over the apex of the 
young leaf 1 . The transverse growth of the ligule is also 
considerable, so that it exceeds the young leaf in width. 
The velum arises from a band of tissue, the sella (saddle) 
1 The development of the ligule and its mature structure are fully described in the 
papers of Farmer, Campbell, and Wilson Smith. 
G g 2 
