490 



BOTANY OF THE LIVING PLANT 



tions (Fig. 3/8). 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 appearance. 



The tracheide of the Fern resembles that of the Pine in being of spindle- 

 form, with its thickened lignified walls marked by bordered pits. (See p. 530.) 



Fig. 379. 



Tracheides of Pteridinm. A =the end and 

 about one third of the length of a tracheide with 

 part of the lateral wall in surface view, showing 

 scalariform marking ( x ioo). Z?=part of A 

 magnified 200. C«=thin longitudinal section 

 through a lateral wall where two tracheides 

 adjoined ( v 375). D= similar section through 

 oblique wall at / ( x 200) . there the pit mem- 

 branes are not visible. (After De Bary.) 



Fig. 380. 



Sieve-tubes of Pteridium. .4= end 

 of a tube separated by maceration 

 ( x 100). B = longitudinal section 

 through phloem showing one sieve- 

 tube with the sieve-plates (sj) in surface 

 view, c, c are walls shown in section, 

 bearing sieve-pits ( x 200). 



But whereas the pits in the Pine are circular, those in the Fern are liable to be 

 transversely elongated, as is natural in tracheids so wide as these are. 

 Their features are well seen in longitudinal sections, but better if they are 

 isolated by maceration (Fig. 379, A). The elongated pits he parallel to one 

 another, and this is specially well seen where two wide tracheids have faced 

 one another. From the ladder-like appearance that results they have been 

 called scalariform tracheids. Examined under a high power the double out- 

 line of the pits is seen, and when the pits are small and circular the similarity 

 to those of the Pine is plain (Fig. 379, B). In most Ferns the pit-mem- 

 branes persist, but in Pteridium they appear to be liable to be broken down, 

 and the cavities thrown together, technically as in vessels. The tracheides 



