116 PROF. BOWER : 
Leptosporan giates, where the spore-numbers are relatively small, the number 
of sperms is smaller also. Such facts give confidence in the value of this 
. criterion, indicating that the sporangium is a real index of relative com- 
plexity of organization, which is thus seen to extend to the gametophyte 
generation, 
In similar ways other criteria have been established. Each has been 
checked by comparison, and as far as possible according to the fossil 
evidence. The most important may be tabulated as follows for the Filicales, 
i.-xiii. relating to the sporophyte, xiv.-xvii. to the gametophyte :— 
i. External form, the upright radial shoot being relatively primitive, 
the prone derivative, but with frequent reversions to the upright. 
ii. Cellular segmentation, the more complex being primitive, and the 
less complex derivative. 
iii. Dermal appendages, simple hairs being primitive, and scales de- 
rivative. 
iv. Stelar structure, the protostele being primitive, and successively 
the medullated protostele, the Lindsaya-type, the solenostele, and 
the dictyostele being derivative. The progression may in certain 
cases be followed in the ontogeny. 
v. Leaf-trace, the coherent being primitive, and the divided trace 
derivative. This may also be followed in the ontogeny. 
vi. Venation, the open venation being primitive, and the reticulate 
derivative. With this goes the “webbing” of the leaf to form 
large laminar areas. 
vii. Soral position, the marginal being the prevalent primitive type, and 
the superficial frequently, or perhaps always derivative. 
viii. Soral construction, the simple simultaneous sorus being primitive, 
the gradate a frequent middle condition, and the mixed sorus 
derivative. 
ix. Indusial protections, none being present in primitive types: various 
in form in intermediate types : but again indusia are absent in the 
most advanced. 
X. Sporangial structure, eusporangiate being primitive, and lepto- 
sporangiate derivative. 
xi. Mechanism of dehiscence: the annulus is indefinite in structure, with 
median dehiscence in primitive types: oblique and continuous, 
with lateral dehiscence in intermediate types: vertical and inter- 
rupted, with lateral dehiscence in advanced types. 
xii. Spore-output : large (i.e., typical numbers 128, 256, 512, 1024, &e.) 
in primitive types: small (e.g., 64, 48, 32, 24, 16, 8, &c.) in 
derivative types. 
xii. Character of spore: the form is not a reliable character, except in 
