434 Scott and Hill.—Strue hire of Isoetes Hystrix. 
appear to have the typical structure. Fig. 24 shows, in 
vertical section, a transverse sieve-plate, which is thickened 
by a callus-deposit. Sieve-areas also occur on the lateral 
walls, and on these also, swollen masses, giving the reactions 
of callus, are formed (Fig. 25). 
Towards the sheathing base of the leaf the structure 
changes considerably. The mechanical strands die out, the 
stomata diminish in number and ultimately cease, and the 
mesophyll here contains little or no chlorophyll. At the level 
of the glossopodium the four cavities of the mesophyll come 
to an end. Lower down, two new cavities appear, one on 
each side, and at the level of the sporangium these cavities 
take up a position in the wings of the leaf, on either side 
of the velum. As regards the vascular bundle, the number 
of xylem-elements increases in the centripetal direction; 
the three intercellular spaces of the xylem are extremely 
conspicuous in the region immediately above the base of the 
ligule. Still lower down, opposite the sporangium, the 
structure of the bundle undergoes a more important change. 
No intercellular spaces are formed in the xylem, which here 
consists of a considerable number of small tracheides, inter¬ 
mixed with parenchyma. Kruch (1890, p. 76) states that 
the xylem in this region, and in the leaf-traces, is not always 
entirely centripetal, but that some elements are added towards 
the phloem. This statement is confirmed by our own ob¬ 
servations, which in some cases showed developing tracheides 
between the first-formed xylem-elements and the phloem. 
Thus the foliar bundle towards its base tends to pass from 
exarch to mesarch structure. 
The median protophloem is persistent in this part of the 
leaf, so that the layer of sieve-tubes forms a continuous arc 
when mature, and is not divided into two lateral groups as in 
the lamina, higher up. 
The base of the deep-seated ligule is surrounded by a very 
conspicuous sheath of tracheides, as described by many 
previous writers (see Fig. 27). These tracheides, which are 
developed early, are especially abundant on the ventral side 
