CH. IX] 



HISTOLOGY. 



127 



sieve-tubes. In the fern the sieve-tubes differ from the 

 typical sieve-tubes of the flowering plants in which a large 

 transverse sieve-plate separates two contiguous elements. 

 In the fern the sieve-plates are on the longitudinal walls, 

 and are therefore most easily seen in longitudinal section. 

 Fig. 55 shows irregular elongated areas of a granular, or 

 faintly dotted aspect, which are the sieve-plates pierced 

 by strands of protoplasm passing from one element to the 

 next. 



Inside the phloem are the vessels of the xylem im- 



scl 



spy 



:T-T/ seal. 



sv.t. 



FIG. 56. 



MACERATED RHIZOME OF Pteris, 

 showing the isolated elements. 



scl, sclerenchyma. seal, scalariform vessels. 



sp.v, spiral vessels. sv.t, sieve-tubes. 



