THE PTERIDOPHYTES. 



191 



that have resulted from the differentiation of the cells of 

 the apical meristem. Starting from the centrally situated 

 ground parenchyma, and passing either 

 to the top or the bottom of the rhizome, 

 make out the following succession of 

 tissues : 



a. A fibro vascular bundle. The row 

 of cells which separates this from the 

 ground parenchyma is known as the 

 bundle sheath. Then come sieve tubes 

 with the surfaces marked by irregular 

 areas of perforations. Next are wide 

 scalariform vessels with distinct mark- 

 ings somewhat resembling the rungs of 

 a ladder. In the center of the bundle 

 are spiral vessels, i.e., vessels with the 

 cell Avail strengthened by a band of cel- 

 lulose running spirally around on the 

 inside. These will not be seen, of 

 course, unless the section passes through 

 the center of the bundle. Outside the 

 spiral vessels the order is reversed, and 



scalariform vessels, sieve 

 tubes, and bundle sheath 

 are again seen. 



b. Next outside the 

 fibrovascular bundle is 

 another region of ground 

 parenchyma. Follow- 

 ing this is a region of 

 the thick-walled cells 

 of the sclerenchyma. 

 Then more ground pa- 

 renchyma, which may happen to include another fibro- 

 vascular bundle. Then come the thick- walled cells of the 



FIG. 114. Vertical 

 longitudinal sec- 

 tion of the apical 

 bud of Pteris aqui- 

 lina, showing the 

 apical cell at A in 

 process of divid- 

 ing. (X480.) 



FIG. 115. a.c., an apical cell of Pteris 

 aquillna seen in vertical transverse sec- 

 tion. The apical bud was on the point 

 of dividing, and a second apical cell is 

 seen at I. (After Sachs.) 



