STRUCTURE OF THE SPOROPHYTE 



by an incomplete ring of sclerenchyma. The outer series correspond to the 

 mesh-work of Dryoptci'is {Neplirodiuvi), the inner are accessory or medullary 

 meristeles (Fig. 3). Each one is a compact body of vascular tissue, and is 

 circumscribed by a complete endodermal sheath, which delimits it sharpl>' 

 from the surrounding parenchyma. The details are shown respectively in 

 transverse and in longitudinal section in Figs. 4 and 5. Each meristele 

 consists of a central core of xylem, surrounded by phloem. A small part 

 of one of them, examined either in transverse or in longitudinal section 

 under a high power, gives the following succession of tissues (Figs. 4, 5). 

 Passing inwards from the starchy ground-tissue, with intercellular spaces {g). 



Fig. 4. Part of a transverse section of a meristele of Bracken. ^= ground 

 parenchyma. £; = endodermis. //z = phloem with sieve-tubes. xy = xylem, 

 with large scalariform tracheides. Some smaller tracheides lying centrally 

 are the protoxylem. Note that no intercellular spaces are seen within the 

 endodermis. ( x 75.) 



the layer of brownish cells of the endodermis ie) forms a continuous barrier, 

 delimiting the strand sharply. Within it follows the pericycle, with its cells 

 not very regularly disposed, but corresponding roughly to the cells of the 

 endodermis, both having been derived by division from a single layer. 

 Within this comes the phloem (ph), with large sieve-tubes as the character- 

 istic elements. They are thin-walled, with watery contents. The lateral 

 walls where two adjoin bear the sieve-plates, and are recognised by glistening 

 globules that adhere to them. They are embedded in parenchyma, which 

 extends inwards into the xylem, and may be called collectively conjicnctive 



