I] STRUCTURE OF THE SPOROPHYTE 7 



circular, those in the Fern are liable to be transversely elongated, as is natural in tracheides 

 so wide as these are. Their features are well seen in longitudinal sections, but better if 

 they are isolated by maceration (Fig. 6, A). The elongated pits lie parallel to one another, 

 and this is specially well seen where two wide tracheides have faced one another. From 

 the ladder-like appearance that results they have been called scalariform tracheides. 

 Examined under a high power the double outline of the pits is seen, and when the pits 

 are small and circular the similarity to those of the Pine is plain (Fig. 6, B). In most 

 Ferns the pit-membranes persist, but in Pteridium they appear to be liable to be broken 

 down, and the cavities of adjacent tracheids thrown together as they are seen to be in 

 vessels. The tracheides of the protoxylem are seen in longitudinal section to be spiral or 

 reticulate, as in other Vascular Plants (Fig. 5). 



Fig. 6. Tracheides of Pteridium. A = the end 

 and about one-third of the length of a tra- 

 cheid, with part of the lateral wall in surface 

 view, showing scalariform marking ( x loo). 

 j9 = part of ^ magnified 200. C=thin longi- 

 tudinal section through a lateral wall where 

 two tracheides adjoin (X375). Z' = similar 

 section through oblique wall at / ( x 200). 

 There the pit-membranes are not visible. 

 (After De Bary.) 



Fig. 7. Sieve-tubes of Pteridium. 

 A = end of a tube separated by 

 maceration (xioo). ^ = longi- 

 tudinal section through phloem 

 showing one sieve-tube with the 

 sieve-plates (j'j) in surface view. 

 c, c are walls shown in section, 

 bearing sieve-pits ( x 200). 



The sieve-tubes are also spindle-shaped, and are without companion-cells. Their cellu- 

 lose walls are swollen. Where two sieve-tubes adjoin, numerous thinner sieve-areas of 

 irregular outline are borne. They are found to be perforated by very fine protoplasmic 

 threads extending between highly refractive globules that adhere to the walls (Fig. 7). 

 Such tracheides and sieve-tubes are characteristic of Ferns and, with differences of detail, 

 of other Pteridophytes as well. 



The anatomy of the leaf in Ferns resembles that of Seed-Plants down 

 even to the collateral structure of the vascular strands. Being chiefly shade- 

 loving plants chlorophyll is usually present in the cells of their epidermis,. 



