304 



GENERAL BOTANY 



Skeletal strands 



Bad 



(liriilcil 



a single strand, and the conversion of the solid vascular bundles 

 by an increasing number of leaf gaps, is also shown in the figures. 

 At the left in c and d a root is being given off. 



The minute structure of the tissues of ferns is strikingly 

 shown in the large-celled tissues of Pteris. Each vascular 

 bundle is concentric, as in the maidenhair, with a central xylem 



mass surrounded by a 

 sheath of phloem. 



In long sections the 

 phloem is seen to be 

 composed of large sieve 

 tubes having numerous 

 sieve plates, together 

 with considerable phloem 

 parenchyma (Fig. 174). 

 The xylem is made up 

 of single-celled water- 

 conducting elements, or 

 tracheids, which have the 

 wall markings of ducts. 

 These tracheids in Pteris 

 and some other ferns 

 often unite end to end 

 to form primitive water 

 ducts. Such a primitive 

 duct, with a ladderlike 

 (scalariform) arrangement of the thick and thin places of the 

 cell wall, is shown in Fig. 174. A large sieve tube, with innu- 

 merable sieve plates of varying form and size, is also shown. 

 Both the duct and the sieve tube are flanked by living storage 

 parenchyma cells. The tissues of the cortex and epidermis are 

 similar in arrangement and structure to those of the maiden- 

 hair. The cell walls of the epidermis and skeletal tissues are 

 greatly thickened, and the cells of the latter are elongated and 

 are fibrous in their nature. These two tissues, together witli 

 the inner skeletal strands, form an admirable protective and 

 strengthening exoskeleton for the fern rhizome. 



FIG. 173. Diagram illustrating the development 

 of the stem tissues in Pteris aquilina 



a, section through a young rhizome ; b, section 

 through an older portion of a rhizome, showing the 

 hud which forms the two internal bundles; c, d, 

 older stages in the development of the internal bun- 

 dles and the skeletal tissue. After Jeffrey. Re- 

 drawn from Jeffrey's " Anatomy of Woody Plants " 



