I902J 



EVOLUTION- OF VASCULAR TISSUE OF PLANTS 



217 



The next stage in general evolution, as the writer holds, is 

 that in which a pith arises in the center of the solid stele [fig* -?.), 

 on the outer limit of which an endodermis, or starch-sheath, 

 may or may not be present ; this is seen in such forms as Platy- 

 zoma (Gleicheniaceae,) Schlzaea, the Ophioglossaceae, and in the 

 fossils Zygopteris and Anachoropteris 



(Botryopterideae). The possibility, 

 however, of these forms having been 

 reduced from type 3 is not excluded. 

 In all these ferns the protoxylem, or 



p/i 



—X 



.//■/// 



first-formed portion of the wood, is 

 either situated within the metaxylem, 

 or later-formed portion, a short dis- 

 tance from the periphery, or else, as 



in Lygodium, quite at the periphery; Fig. 2.— The tubular stele 

 that is to say, the xylem is chiefly "^^^^ central pith: x,px,ph,^,% 



ce?itripetal in its development. 



The solenostele is the name given to the third stage of differ- 

 entiation, in which to the internal endodermis is added an internal 

 ^one of phloemi^fig. 3 ,\\ this is found in Matonia,Loxsoma, Aneimia 



-px 



mfig. /. 



Mexica?ia (Schizaeaceae), and in 

 a few other plants, such as Medid- 

 losa stellata among the Cycadofi- 



lices. 

 ^ The dialystelic condition ap- 

 pears to have been the next step 

 onward, in which the tubular 

 solenostele becomes split up into 

 a number of secondary solid 

 steles or concentric strands 

 [fig, 4), the protoxylem, as in 

 the previous stages, being situ- 

 ated at or near the external limits of the xylem. This structure 

 is directly due to the crowded arrangement of the leaves on the 

 stem, so that frequent gaps in the original solenostele become 

 inevitable in order to allow of the passing out of the leaf-trace 



Fig. 3. — The solenostele: reference 

 letters as before. 



