438 



VA SCULAR CRVPTOGA MS. 



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 Hymenophyllaceae, Osmundaceae, and Cyatheaceae 

 the latter only occur, in the other families sometimes the one kind and some- 

 times the other. 



The spores of many Polypodiaceae are distinguished by the long period 

 during which they retain their power of germination, and by the slowness of this 

 process; those of Hymenophyllaceae often begin to germinate while still in the 

 sporangium. 



Fig. 306. — Development of the spores of Aspidiicr,i Filix-, 



{X 550). 



(a) Histology^. With reference to the Epidermis, attention has been directed on p. 105 

 to the peculiar mode of development 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 'uulgare, 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- 

 folium 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 



^ [For further details see de Bary, Vergleichende Anatomic der Phanerogamen und Fame, 1877.] 



