SELAGINELLA SPOROPHYTE 267 



4. The lacunar tissue, in which the outer paren- 

 chyma, consisting of short and small cells, is to be 

 distinguished from the inner cells, which are elongated 

 in a radial direction, and show that peculiar median 

 constriction above noted in the transverse sections. 



5. The peri cycle, consisting of elongated parenchy- 

 matous cells, with cellulose walls, and often containing 

 chlorophyll. 



6. The phloem, the most prominent elements of 

 which are long narrow elements with cellulose walls 

 and sparing contents : these are regarded as the repre- 

 sentatives of the sieve-tubes. 



7. The xylem, the most prominent elements oi 

 which are spiral and scalariform tracheides, similar 

 to those to be described below as composing the xylem 

 of the bundle in Lycopodium and in the Ferns. The 

 walls are lignified and thickened, and marked by 

 elongated pits, which by their regular arrangement 

 give the scalariform character to these elements. 



V. Cut transverse sections of a rhizophore : mount 

 as before, and observe that the peripheral tissues are 

 not unlike those of the stem, and are marked off from 

 the central cylinder by a somewhat irregular bundle- 

 sheath or endodermis ; that the arrangement of 

 tissues of the central cylinder differs both from that of 

 the stem, and that usual for root-structures, there being 

 but one group of protoxylem (monarch), which is 

 placed laterally: the later-formed xylem forms 

 together with it a central mass, which is surrounded by 

 phloem except at the point opposite the protoxylem. 

 The structure of the individual vascular elements is 

 similar to that in the stem. 



