PLANTS OF THE INDIAN DESEET. 279
Structure of the Segment. — The epidermis of the frond-segment
on the upper surface (fig. 360) consists of sclerotic cells with outer
walls toothed in the middle. The sclerotic epidermis is accompanied
by a sub -epidermal tissue of stone-cells which have the same size and
structure as those of epidermal sclerotic cells. The sub-epidermal
stone-tissue stops short a little behind the margin which is curved
downwards over the sporangia forming a kind of an indusium. The
margin presents a filamentous structure in T. S. and is strengthened
by a spiral thickening of the wall. The lower epidermis is formed of
tabular cells with the outer walls thickened and cuticularised ; there
are groups of stone-cells intercalated amongst epidermal cells above
the stele. The cuticle of lower epidermal cells is also toothed in the
middle of the cells.
Structure of the Stipe. — The epidermal cells of the stipe (fig. 361) are sclerosed, except at the angular portions on the upper side. The sclerosed epidermis is further strengthened by a sub-epidermal tissue of stone-cells. The epidermis at the angular portions is formed of chlorenchymatous cells with outer walls thickened. The outer walls of the sclerosed epidermal cells are toothed in the middle of the cell. The abundant development of epidermal and sub-epidermal scleren- chyma has chiefly the function of reducing transpiration which is very vigorous in Ferns ; and it is a result of the deficiency of water.
Stomata occur on the lower surface of the segment and in the angular portion of the stipe, and are accompanied by ordinary epidermal cells. The guard-cells are much elevated and the front cavity is situated above the surface.
Hairy covering is absent on the segment. On the stipe there occur long unicellular thick-walled hairs strengthened in the basal portion by the spirally thickened wall (fig. 261). The external glands are not found on the segment or on the stipe.
The assimilatory tissue in the segment as well as in the stipe is formed of chlorenchyma. In the stipe there are circular spaces in T. S. one near each margin.
The stele in the marginal portion from where the sporangia origi- nate is larger than those in other portions of the pinnule. Steles, except those at the margin, are strengthened on the lower side by groups of stone-cells intercalated amongst epidermal cells (fig. 360). In the stipe there is a single larger stele in the centre.
