SPOROPHYTE. 25 



maturity, leaving- only the oval basal cell to show where they stood. The 

 hairs are of the same character as those on the lamina. They will be 

 described later. 



Beneath the epidermis is a layer of brown, thick- walled sclerenchyma, 

 as already mentioned (cf. fig. 69). In the petiole proper these cells are 

 long*, and tapering at each end. Near the node they become shorter until 

 they reach the dimensions of the cells of the sclerotic cortex of the stem, 

 with which they are continuous. They may also unite here with a rod of 

 similar tissue derived from the medulla of the stem and lying in the trough 

 of the leaf -trace (cf. p. 21). About the middle of each side of the petiole 

 the sclerenchyma is interrupted by an extension of ordinary parenchyma 

 out to the epidermis (figs. 69, 101). Stomata occur along this line to 

 admit interchange of gases between the intercellular spaces of the paren- 

 chyma and the outside air. For a short distance near the surface of the 

 ground the sclerenchyma is not so interrupted and stomata do not occur; 

 but at about 1 cm. from the rhizome the stomata and parenchyma begin 

 again. The area occupied by them here is easily seen, because the walls 

 of the sclerenchyma cells are of a deep-brown color, while those of the par- 

 enchyma are colorless. This area may end 0.5 to 2 mm. above the rhizome, 

 or may continue as a visible ridge on the posterior side of the leaf -base 

 for 1 mm. or more along the surface of the node. The guard-cells of 

 these stomata stand above the level of the surrounding epidermis. Such 

 lines of parenchyma are common in fern petioles, but by no means universal 

 (Thomae, 1886). 



From the epidermis inward, the cells become larger in diameter, shorter, 

 less acute at the ends, and thinner walled. Two to ten cells in, according 

 to height and age of section, we find soft parenchyma, with large watery 

 cells bearing chlorophyll. These become smaller again next to the vas- 

 cular bundle. They are in contact with the endodermis all around the 

 bundle. This layer has many intercellular spaces in its middle portion. 



The bundle itself is shaped like a trough, opening toward the upper 

 surf ace of the leaf. In transverse section near the rhizome (figs. 82, 100, 101) 

 it is U-shaped, or very nearly like the periphery of a semicircle. Its thick- 

 ness is about the same all round. Higher up the petiole it is more V-shaped, 

 flattened at the angle (fig. 68), and swollen at the tips of the short arms. 

 The endodermis bears the typical Caspary's band. The contents of its 

 cells take up stains with avidity, and probably include mucilage. A par- 

 enchymatous layer within the endodermis may be termed pericycle. Of 

 two cells thickness in most places, it thins off to one cell around the tips of 

 the arms of the bundle, and becomes three-celled on the outer side of each 

 arm (fig. 68). At the latter place, also, the middle cells enlarge greatly to 

 form a very peculiar open tissue. The thin band of xylem in the midst of 

 the bundle follows the latter in general shape, but is more curved in its 

 outlines. At the ends of the U or V it approaches very near to the peri- 



