F1LICINEAE. HOMOSPOROUS FILICINEAE. 



223 



lying among them, as in Pteris aquilina (Fig. 172). The xylem in the concentric 

 bundles (Fig. 171) is surrounded by the phloem containing parenchymatous cells as 

 well as sieve-tubes. In Fig. 172, which is a transverse section from Pteris aquilina, 

 outside the xylem there is first of all a layer of parenchymatous cells containing starch, 

 next the large sieve-tubes (sp\ then a zone of narrow, thick-walled elements, which are 

 either some peculiar form of cells (bast-fibres of Dippel) or must be reckoned with the 

 sieve-tubes (protophloem of Russovv), then another layer of parenchyma containing 

 starch, and lastly the bundle-sheath or endodermis (sg] ; the latter cell-layer forms a 

 very distinct boundary line round the vascular bundle, and consists of compressed cells 

 with walls which early become transformed into cork and have usually a brownish 

 colour. This layer and the layer of parenchyma next inside it are the product of a 



FIG. 172. A fourth part of the transverse section of a 

 vascular bundle in the stem of Pteris aquilina with the sur- 

 rounding parenchyma PP containing starch ; sg the endo- 

 dermis, b the bast-fibres or protophloem of Russow, sp the 

 large sieve-tubes, qq the wide vessels thickened in a scalariform 

 manner, s a spiral vessel surrounded by cells containing 

 starch, K thickened wall of vessel between the pits. Magn. 

 300 times. 



FIG. 173. Pteris aquilina. A extremity of a sieve-tube 

 isolated by maceration. B piece of a longitudinal section ; 

 s\ and Jj sieve-tubes roughly bisected, s\ is bounded on the 

 right by cells of the parenchyma, on the left by s-z ', the whole 

 of the smooth-walled posterior side of s 2 adjoins parenchyma, 

 (the nucleus is shown in two of the cells) ; cc transverse section 

 of the walls bearing sieve-pits. From De Bary, Vergl. Anat. 

 A magn. 142, B 375 times. 



single layer of the fundamental tissue which afterwards divides into two. In smaller 

 bundles the sieve-tubes are often not easy to recognise; in Pteris aquilina (Fig. 173) 

 they have long narrow points and sieve-plates on the side walls. It was said above 

 that Osmunda has collateral bundles, which are seen in a transverse section to be 

 arranged in a circle and to be separated from one another by parenchymatous tissue. 

 The phloem-regions are on the outside toward the circumference and are connected 

 together into a circular zone. 



Through the young plants in all Ferns runs a single axile bundle which is a sym- 

 podium of the leaf-trace bundles ; the first bundle, which generally comes to an abrupt 

 termination in the foot of the embryo, runs a short distance through the stem and then 



