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. 



862-37 



