THE PTERIDOPHYTA : FILICALES, THE FERNS 



559 



Starting from this simple condition, it has been necessary, especially in 

 larger plants, to increase the area of the stele. 



One way of doing this is by the extension of the xylem mass radially 

 outwards into a series of wdngs or flanges, with the protoxylems at the 

 extremities. This is called an actinostele (Fig. 564, c). It is characteristic 

 of many of the early fossil Pteridophyta and survives in some species of 

 Lycopodium and Psilotuni. 



Another method has been to increase the diameter of the stele by the 

 admixture of parenchvmatous cells with the tracheids (Fig. 567), which later 



Fig. 566. — Botryopteris cylindrica. Protostele enlarged 

 showing mesarch protoxylems and surrounding 

 phloem zone. 



results in the central part of the xylem, becoming entirely replaced by a 

 parenchymatous medulla or pith. 



In this way the solid protostele is converted into a tubular structure with 

 a central medulla, the siphonostele. The first condition is illustrated by the 

 fossil type, Melaclepsydropsis duplex, in which the inner zone of the stele 

 consists of a mixture of tracheids and parenchyma, that is to say, a " mixed 

 pith." This type may therefore be regarded as intermediate between a true 

 protostele and a stele with a completely parenchymatous pith. Fossil members 

 of the Osmundaceae also illustrate the complete transition to a medullated 

 stele (Fig. 568). Botrychiiim also illustrates a type in which the pith has 

 developed within the stelar tissues. The next stage is the development of an 

 endodermis between the tubular stele and the pith, forming what is termed a 

 solenostele (Fig. 564, g). 



Very soon after the evolution of this type of stele we find that the leaf 

 trace becomes more elaborate, and instead of merely separating off a small 



