6l2 



A TEXlTBOOK OF THEORETICAL BOTANY 



chloroplasts. Similar cells form the central tissue of the leaf. Inside the 

 assimilating layer is a broad band of sclerotic cells and within these again 

 lies a zone of thin-walled parenchyma. 



The endodermis round the stele is well marked. In the centre of the stele 

 is a core of sclerenchyma cells, round which lies a narrow band of scalariform 

 xylem elements, with the spirally thickened protoxylems projecting outwards 

 as radial ribs. This is a siphonostelic type of structure, though the pith is 

 sclerenchymatous, not parenchymatous, as is usual in Ferns. The phloem 

 is of very indefinite character. Tubular cells are present, but they have not 

 the structure of sieve tubes and are called by the non-committal name of 





.-o^n^v^^ 





• Endophytic Fungus 



/':'^J^/ 







,A 





tM^. 



Fig. 623. — Psilotum triquetritm. Transverse section of 

 rhizome with mycorrhizal cortex and small stele with 

 central xjdem group. 



leptoids. In addition this zone contains groups of cells with their walls 

 thickened, especially at the angles. These cell groups lie opposite the xylem- 

 ribs, but their nature is unknown. In the upper part of the stem the central 

 group of sclerenchyma is not formed and there are no xylem ribs, so that the 

 structure here is typically protostelic. 



The traces which go to the sporangiophores consist of spiral elements 

 given off from the protoxylems ; although small they show a distinct mesarch 

 structure. As mentioned above the leaves receive no vascular tissue. Only 

 the branch traces make any gap in the xylem ring. 



The stele of the rhizome has no pith and the xylem mass is usually rounded 

 in outline, often lacking any protoxylem or ribs (Fig. 623). There is very 

 little phloem and the endodermis is normal. Outside the endodermis are 

 three or four layers of thick, brown-walled cells, as in a Fern root, while the 

 bulk of the cortex is thin-walled and mycorrhizal. All the cells of the surface 



