THE LIFE-HISTORY OF A FERN 



[CH. 



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 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. 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 

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

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



Fig. 



Longitudinal h,ection of meiistele of Bracken. Lettering and 

 magnification as in Fig. 4. 



A transverse section (Fig. 4) gives only one aspect in which such com- 

 plicated tissues can be studied. Its interpretation is aided by longitudinal 

 sections (Fig. 5). It is then seen that the sieve-tubes, which are elongated 

 and pointed, bear their numerous sieve-areas upon the lateral walls : and 

 that the spindle-shaped tracheides bear also upon their lateral walls those 

 transversely elongated pits which give them the so-called scalariform appear- 

 ance. Interspersed between the tracheides and sieve-tubes are cells of the 

 conjunctive parenchyma, and the meristele is delimited by endodermis {e), 

 as seen in the transverse section. 



The tracheide of the Fern resembles that of the Pine in being of spindle form, with its 

 thickened lignified walls marked by bordered pits. But whereas the pits in the Pine are 



