CHAPTER XI 



DERMAL AND OTHER NON-VASCULAR TISSUES 



While in Ferns, as in other Vascular Plants, priority of importance attaches 

 to the vascular system by virtue of its greater " phyletic inertia," other tissues 

 which envelope it make up a great part of the body of the sporophyte. 

 They may be grouped as Ground-Parenchyma, Sclerenchyma, and Epidermis. 

 Associated with the last are the Dermal Appendages, that is, Hairs and 

 Scales, which arise as outgrowths of the surface. Exclusive of the dermal 

 appendages these tissues prove to be more directly variable according to 

 external conditions than the vascular system, and accordingly they are less 

 trustworthy for purposes of phyletic comparison. 



Parenchyma and Sclerenchyma 

 The Ground-Parenchyma consists for the most part of living cells of 

 more or less rounded form, with walls of very variable thickness, and with 

 large intercellular spaces. Usually it is functional in the leaf-blade as photo- 

 synthetic tissue, and in the stem as storage-parenchyma. The last is naturally 

 most marked where a seasonal leaf-production occurs, the leaves dying off 

 and being renewed annually, as in Pteridium. Starch is the common storage- 

 material, sometimes with mucilage (compare Figs. 131, 173). The inter- 

 cellular spaces often show in great perfection pectose-rods traversing them, 

 in the narrower angles even extending from wall to wall. These are particu- 

 larly well seen near the leaf-base in the Marattiaceae. The cell-walls are 

 frequently colourless, but in the leaf-stalk and stem of Ferns they often 

 show varying tints of yellow or brown, even when they are relatively thin. 

 Where the walls are thick the colouring may be deep brown or almost black, 

 while the cytoplasm and storage-contents may still be seen in them. There 

 is, in fact, no sharp distinction between thin-walled and sclerotic cells. More- 

 over the distribution of the hard tissue-masses is often irregular, giving the 

 impression that in the ontogeny any cell may be determined for development 

 either as a thin-walled or as a sclerotic unit. The form of the sclerotic cells 

 varies from rounded or oblong stone-cells to elongated fibres. For instance 

 the former are seen in the stony rind of the rhizome oi Pteridium (Fig. 3): 

 they are heavily sclerosed, with pitted walls, and without starchy contents. 

 But it is not so much the nature of the walls as their form which marks them 

 off from the parenchyma of the cortex, into which they merge internally. 

 The sclerotic fibres compose those deeply seated tracts of brown scleren- 

 chyma, whose spindle-shaped cells are marked by oblique slit-like pits in 



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