FERNS. 



359 



hibited, reference must be made to the work of Rees already quoted ; the illustra- 

 tions, Figs. 266 and 267, had already been drawn on the wood when his more 

 detailed investigations were pubHshed, and confirm his statements on all essential 

 points. It is impossible, as has already been maintained, that the mother-cells of the 

 spores can arise by free cell-formation. Each mother-cell is, in Aspidium Filix-mas 

 (Fig. 268, /), provided with an evident nucleus; after its absorption (//), two new 

 large clear nuclei arise (///), between which an evident line of separation is some- 

 times to be seen. After the absorption of these nuclei which show the commence- 

 ment of a bipartition, four new smaller nuclei appear {IV), the mother-cell splitting 

 up into four spore-cells ( T), the relative position of which varies (as is shown in 

 Figs. VI, VII, and VIII), The spore now becomes clothed with its cell-wall, 

 which is differentiated into an endospore consisting of cellulose and a cuticularised 

 brown exospore furnished with ridges {IX), and chlorophyll is formed within the 

 spore. 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 Hymenophyllacese often begin to germinate while still in the 

 sporangium. 



Fig. 268.— Development of the spores oi Aspidium Filix-mas (XS50). 



Ihe Systematic Classification of P'erns, as generally given in handbooks, is based arti- 

 ficially on the form of the mature sporangium for the families, and of the sorus for the 

 genera ; only those groups which have already been repeatedly meniiioned are accurately 

 known. It appears certain that the Hymenophyllaceae contain the lowest forms most 

 nearly allied to the Muscineae ; the genetic relationship of the other families with the 

 Hymenophyllaceae and with one another has not yet been ascertained; but the Hy- 

 menophyllaceae probably form the starting point for two or more series of families. 

 IMettenius (Filices Horti Botanici Lipsiensis) distinguishes the following families, which 

 I adopt with some alteration in the arrangement : — 



I. Hymenophyllaceae. 



2. Gleicheniacese. • 5. Marattiacese (vide infra). 



3. Schizaeaceae. 6. Cyatheaceae. 



4. Osmundaceae. 7. Polypodiaceae. 



In the characters of the families which follow, I have adopted the diagnoses of 

 Mettenius, but at the same time give prominence to a few facts which may serve to 

 complete the morphological statements already made. 



I. Hymenophyllacese. The sporangia have an oblique or transverse complete 

 annulus ; and therefore burst with a longitudinal slit ; they are formed on a prolon- 

 gation of the fertile vein (the Columella), projecting beyond the margin of the leaf, 



