Metaphyta Pteris. 119 



much crushed, the walls being considerably thickened. The 

 vessels are of peculiar form and deserve special notice. In 

 section they appear as large apertures lying internal to 

 the bast fibres, and often surrounded by the starch-bearing 

 parenchyma above referred to. Each vessel is an elongated 

 tube whose walls are riddled with minute apertures. These 

 apertures are not uniformly distributed, but are collected in 

 areas, the so-called sieve plates (see fig. 68). From their 

 possession of these curious discs the vessels of the phloeir 

 are known as sieve tubes. Enclosed by the phloem lies a 

 mass of parenchymatous tissue surrounding the vessels, and 

 collectively known as the xylem or wood. The paren- 

 chyma does not differ from specimens of that tissue already 

 described. The vessels are complicated in structure, and 

 are known as tracheae. The wall of each has laid down 

 upon it internally a layer of secondary deposit, and that not 

 uniformly, but in the shape of transverse bars. These bars 

 are joined together at their ends ; hence a series of slit-like 

 spaces are left where the primary cell-wall can be seen. 

 The appearance of such a vessel reminds one forcibly of a 

 ladder, the thickenings corresponding to rungs : hence the 

 name given to these vessels, viz. scalariform (fig. 48). 

 Associated with the tracheae are found long, narrow vessels 

 in which a spiral thickening has been laid down : these are 

 characteristic of the xylem and are called spiral vessels. 

 They are central in position (fig. 47). The structure of these 

 several elements and their relationship to each other may be 

 best made out by comparing a transverse and a longi- 

 tudinal section of a strand. 



A fibro -vascular strand, then, consists of two kinds of 

 vessels surrounded by certain fibres and cells which strengthen, 

 protect, or act as padding to the vessels, viz. the vessels of 

 the xylem and the vessels of the phloem the tracheae 

 and sieve tubes respectively. These two important elements 

 are differently arranged with reference to each other in 

 different plants. In Pteris the arrangement is said to be 



