XI] HAIRS AND SCALES 197 



often accompanied by sclerotic bands, giving a girder-construction effective 

 for strengthening the expanded blade. Such examples witness to the 

 mechanical use of sclerenchyma in Ferns. 



But on the other hand, less definitely formed sclerotic masses may be 

 found closely following the vascular tracts in stems and roots, but so disposed 

 that mechanical effect is probably not their sole function. Their close relation 

 to the vascular tissues suggests that they act also as reservoirs for water of 

 imbibition, as in the case of many xerophytic Flowering Plants. More especially 

 is this view upheld by the fact that in strongly xerophytic Ferns, such as 

 NotJiolaena, CheilantJies, Pellaea, or Platysoma, the proportion of scleren- 

 chyma is very high. It is in their roots that this is most marked, for there 

 a thick sclerotic band surrounds the stele, being densest just at its boundary 

 (Fig. 9, p. 8). It thus seems probable that the sclerenchyma serves a double 

 function in Ferns: but it is so directly adaptible to circumstances and variable 

 in occurrence that its features have little value for phyletic comparison. 



The epidermis calls for little remark. When mature it resembles struc- 

 turally that of non-specialised Flowering Plants. Chlorophyll is often present 

 in its tabular cells, and the stomata appear at the level of the epidermal layer. 

 Many hygrophilous Ferns have stomata of the " aquatic " type, opening 

 directly by a wide aperture into the air-chamber below. These non-specialised 

 characters go along with the late differentiation of the epidermis at the apex 

 of stem and leaf. It is only after the segments derived from the initial cell 

 or cells have undergone numerous tangential divisions that the dermatogen 

 can be recognised, and traced on as giving rise to the epidermis. 



Hairs and Scales 

 All the families of Ferns bear appendages springing from the epidermis. 

 Even the Hydropterideae possess them in their youngest parts, and the 

 Ophioglossaceae also, though they appear to have naked surfaces when 

 mature. In some of the earliest fossils hairs provide characteristic features. 

 Biologically they give protection to the young developing parts. According 

 to their structure the dermal appendages furnish reliable features for com- 

 parison. The simplest of them are unbranched/^^/rjr, which arise by outgrowth 

 of single superficial cells, and cell-division is only by walls transverse to the 

 axis of their growth. The result is a linear series of cells (Fig. 185, i). Not 

 uncommonly the distal cell develops as a gland. Frequently it secretes 

 resinous material, or essential oil soluble in alcohol, as in Nephrodium inolle, 

 and many others. This is often deposited between the outer and inner 

 layers of the cell-wall. Others again contain mucilage secreted in droplets 

 within the cytoplasm, as in the Osmundaceae, and in Blechimm (Fig. 185, v). 

 The mucilage swells with water, and the secretion of many hairs flowing 

 together covers the part with a protective, water-containing sheath. In others 



