FILICALES 



489 



as the characteristic elements. They are thin-walled, with watery 



contents. The lateral walls where two adjoin bear the sieve-plat 

 which are recognised by glistening globules that adhere to them. They 

 are embedded in parenchyma, which extends inwards into the xylem, 

 and may be called collectively conjunctive parenchyma. The chief 

 features of the xylem (xy) are the tracheides, which are relatively large, 

 with a very characteristic polygonal outline. They have woody 

 walls, and no protoplasmic contents. Where two adjoin the walls are 



Fig. 378. 

 Longitudinal section of meristele of Bracken. Lettering and magnification as in Fig. 377. 



flattened, and of double thickness, showing that each has its own share 

 of the thickening, which overarches the pit-membrane as in the pits of 

 Conifers. The structure is in fact essentially the same, only in Ferns 

 the pits are liable to be extended transversely. But where the 

 tracheide abuts on parenchyma-cells the pits are narrower. Internally, 

 and usually about the foci of the elliptical meristele, smaller tracheides 

 are found. These are the first-formed tracheides, or protoxyUm. 

 The meristele of a Fern is thus concentric in construction ; it is 

 strictly delimited, and has no provision for increase in size. 



A transverse section gives only one aspect in which such complicated 

 tissues should be studied. Its interpretation is aided by longitudinal 



