438 
VASCULAR CRYPTOGAMS. 
and a cuticularised brown exospore furnished with ridges {IX), and chlorophyll 
is formed within the spore. 
In various other Polypodiaceae, according to Russow, the course of the de- 
velopment of the spores is in so far different, that the mother-cell, as occurs also 
in the formation of the pollen in Phanerogams, divides into four thick-walled 
cells, the so-called special mother-cells ; the protoplasm of each of these then 
forms around itself a permanent coat, and the walls of the mother-cell undergo 
absorption. Spores of the shape indicated in Fig. 306 are said to be bilateral 
in contradistinction to those which have been formed from a mother-cell in 
which the four nuclei were placed tetrahedrally, and which have therefore a 
rounded tetrahedral form. In the Hymenophyllacese, Osmundaceae, and Cyatheaceae 
the latter only occur, in the other families sometimes the one kind and some- 
times the other. 
The spores of many Polypodiacese are distinguished by the long period 
during which they retain their power of germination, and by the slowness of this 
process; those of Hymenophyllacese often begin to germinate while still in the 
sporangium. 
(a) Histology'^. With reference to the Epidermis, attention has been directed on p. 105 
to the peculiar mode of developm-ent of the stomata in many cases. It may also be 
mentioned that the epidermal cells usually contain chlorophyll-granules. 
The Fundamental tissue of the stem and of the leaf-stalks consists, in some species 
(as Polypodium aureum and 'vulgare, and Aspidium Filix-mas), entirely of thin-walled 
parenchyma; in others (as Gleichenia, species of Pteris, and Tree-ferns), string-like, 
ribbon-shaped, or filiform portions of the fundamental tissue become differentiated, the 
cells of which undergo great thickening, and become brown-walled, hard, and prosen- 
chymatous, forming sclerenchyma. In the stem of Pteris aquilina (Fig. 307, A) two 
thick bands of sclerenchyma of this description {pr) lie between the inner and outer 
fibro-vascular bundles, and fine threads of sclerenchyma appear on the transverse section 
of the colourless parenchyma as dark points. In other cases (as in Polypodium 'vaccinii- 
foUum and in Tree-ferns), dark layers of sclerenchyma, the nature of which was in 
these cases first correctly recognised by H. von Mohl, form sheaths round the fibro- 
vascular bundles, to which the erect stem more especially owes its firmness. The outer 
layer of the fundamental tissue of thicker stems and leaf-stalks lying beneath the epi- 
dermis is often dark brown and sclerenchymatous, forming a hard firm sheath, as again, 
for instance, in Pteris aquilina (Fig. 307, A, r) and Tree-ferns. In order to facilitate, 
in spite of this firm coat, the communication of the outer air with the inner parenchyma 
Fig. 306.— Development of the spores of AspidiiDn Filix-; 
(XSSo). 
^ [For further details see de Bary, Vergleichende Anatomie der Phanerogamen und Farne, 1877.] 
