FILICINEm. 



439 



which is rich in assimilated food-materials, it is, in Pteris aquilina, interrupted along 

 two lateral lines, where the colourless parenchyma rises to the surface. In Tree-ferns, 

 on the other hand, according to H. von Mohl, depressed cavities appear on the enlarged 

 base of the rachis of the leaf, where the sclerenchyma is replaced by a loose and pul- 

 verulent tissue. 



It may be mentioned here in addition, as an isolated histological peculiarity, that in 

 Aspidium Filix-mas, according to Schacht, roundish stalked glands occur in the funda- 

 mental tissue of the stem, which I have also noticed in the green parenchyma of the 

 leaves, and on the pedicels of the sporangia of the same Fern (Fig. 304, C, d). 



The lamina of the leaf consists in Hymenophyllaceae only of a single layer of cells, 

 as in Mosses ; in all other Ferns it is formed of several layers. Between the upper and 

 under epidermis lies a spongy parenchyma containing chlorophyll, the mesophyll, pene- 

 trated by the fibro-vascular bundles which form the venation of the leaf. The course 



FIG. -^OT.— Pteris aquilina ; A transverse section 

 of the stem, r its brown sheath (the layer of scleren- 

 chyma beneath the epidermis), / the soft colourless 

 parenchyma of the fundamental tissue ; 4'' inner fibro- 

 vascular bundles ; ag upper broad outer bundle ; B 

 the separated upper fibro-vascular bundles of the 

 stem st, and of its branches st' and st", b bundles of 

 the leaf-stalk, u u outline of the stem (natural size). 



Fig. 308.— a quarter of tlie transverse section of a fibro-vascular 

 bundle from the stem of Pteris aquilina, with the adjacent parenchyma 

 P containing starch, sg- the bundle-sheath, b the layer of bast-fibres, 

 sp the large sieve-tubes, gg the large vessels of the xylem thickened in a 

 scalarifonu manner, 5 a spiral vessel surrounded by cells containing 

 starch (x 300). 



of the veins is very various ; sometimes they run branching dichotomously at acute 

 angles, or spreading like a fan upwards and sideways, without anastomosing and without 

 forming a mid-rib ; more often the undivided lamina, or a division of the lobed, incised, 

 or pinnate leaf, is penetrated by a distinct median vein though but slightly projecting, 

 from which spring more slender branches, which themselves again ramify dichotomously 

 or apparently monopodially, and run to the margins. The finer veins frequently anasto- 

 mose like those of the leaves of most Dicotyledons, and divide the surface into areolae 

 of characteristic appearance. 



The Fibro-'vascular Bundles of Ferns are closed; they consist of a mass of xylem, 

 completely enveloped by a layer of phloem. Besides a few narrow spiral vessels, lying 

 at certain definite points in the transverse section, the xylem consists of vessels with 

 bordered pits which usually resemble transverse clefts (scalariform vessels), their ends 



