126 MAIDEN-HAIR FERN. 



ferentiate into the tissues which compose and surround 

 the fibro-vascular bundles. These tissues are quite dis- 

 tinct from each other as well as from the original 

 parenchyma. 



The sheath which incloses the bundles does not 

 belong to the bundle itself, either in the fern or other 

 plants, but to the surrounding parenchyma. 



The apparently perforated plates on the walls of the 

 sieve cells can not be seen clearly because of the layer 

 of protoplasmoid substance which adheres to the walls. 

 The perforations themselves are not easily demon- 

 strated though DeBary " thinks he has seen fine fila- 

 ments connecting granules on opposite sides of a plate. 

 The continuity of protoplasm between other than 

 sieve-cells has been demonstrated in many plants. 



The arrangement of the tissues of the bundles in 

 stems and roots is of different types. In the former, 

 the phloem of the bundle encircles the xylem whence 

 it is known as a concentric bundle. In the latter, 

 the xylem forms a plate dividing the phloem into 

 two portions' which stand one on each side of it. 

 Assuming a center, the xylem and phloem masses are 

 symmetrically disposed about it, whence the bundle is 

 known as radial. 18 



The root-bundle contains a tissue, the pericambium, 

 whose cells are still capable of division ; no such 

 tissue is found in the stem-bundles. New roots have 

 their origin not in the pericambium as in phanerogams, 

 but from cells of the bundle-sheath. 19 



17 Comparative Anatomy, p. 181. 



18 Cf. Strasburger, Das botanische Practicum, p. 209 ; DeBary, Com- 

 parative Anatomy, p. 362. 



19 Cf. Strasburger, Das botanische Practicum, p. 276 ; Prantl and 

 Vines, Text-book of Botany, p. 51. 



