330 MOSSES AND FERNS CHAP. 



In Onoclea struthiopteris, as in most leptosporangiate Ferns, 

 the outer cortical cells become changed into sclerenchyma. 

 The sclerenchyma forms several hypodermal layers, distinctly 

 separated from the inner cortical parenchyma. These scler- 

 enchyma cells are much elongated ; their lateral walls are some- 

 what uneven, and in their younger stages swell up more 

 strongly under the action of potassic hydrate than do the cortical 

 cells. Their walls become thick, are first pale yellow, and later 

 a dark reddish brown. The walls are very markedly striate, 

 and the central lamella distinct. Deep pits extend down to the 

 latter. 



The bundles in the stems of the Polypodiacese are very 

 uniform in structure. They are usually elliptical in section, 

 and the first tracheary tissue formed is a strand of small spiral 

 or reticulate tracheids at the foci of the bundle. From there 

 the formation of the very large scalariform ones, so character- 

 istic of the leptosporangiate Ferns, proceeds towards the centre 

 of the bundle, where the last-formed ones are situated. The 

 young tracheids have thin walls and abundant protoplasm, but 

 as the wall thickens, the contents gradually disappear, and 



B- 



A. 



FIG. 182. Polypodium falcatum; A, Transverse section of the rhizome, X6; B, a sin- 

 gle vascular bundle, Xi7S5 en, endodermis. 



finally no living protoplasm remains in them. Faint elongated 

 transverse pits become evident, and the spaces between these 

 rapidly thicken at the expense of the cell contents until all the 

 protoplasm is used up. The thickened bars between the pits 

 give the characteristic ladder-like appearance to the older 



