XLix] PROTECTION OF GAMETANGIA 267 



be sought among lowly and generalised forms rather than among the more 

 specialised Algal types. Similarities of vegetative form between the larger 

 types and the most primitive Archegoniatae suggest homoplasy rather than 

 homogeny. The comparison of the gametangia stands on a different plane, 

 and the differences can be correlated with the environment. Sub-aerial life 

 entails more exact protection of the gametes and of the zygote than is 

 needed in aquatic life: the reaction of the organism is seen in the cellular 

 protective walls of the gametangia, absent in most Algae, but constantly 

 present in the Archegoniatae, the origin of which may well*have been by 

 sterilisation of superficial cells. Whether or not this was so, the gametangia of 

 archegoniate plants are so protected now, and in particular the archegonium, 

 which is the distinctive female organ of early land-living plants. Its pro- 

 tective function does not end with sjmgamy : the young embryo cannot, as 

 in so many Algae, be cast adrift to take care of itself. It is retained on the 

 parent, encapsulated in the archegonium and nursed there — a matter of the 

 greatest importance for successful sub-aerial life. An internal embryology, 

 however, restricts the development of the embryo: in particular a fila- 

 mentous form, such as is common in freel}' germinating Algae, would be 

 inconvenient. There are evidences of this primitive construction in the young 

 sporophyte of Ferns, and particularly the suspensor in certain primitive 

 types may be held as such : but they have been partially or completely 

 eliminated, and the embryo of most Ferns is from the first a massive 

 structure. 



The production of carpospores in the Bryophyta is carried out without 

 delay in the sporogonium, the necessary nutrition being drawn mostly from 

 the persistent gametophyte: and the capsule is distal. But where the game- 

 tophyte is small and evanescent, as it is in the Ferns, the exigencies of 

 imtrition fall early upon the embryo itself, with the consequence of an 

 interpolated vegetative system, and spore-production is delayed. Still in 

 certain relatively simple vascular plants the distal position of the sporan- 

 gium, which was probably its original place, is maintained (Psilophytales, 

 Sporogonites, Stauropteris). In many primitive Ferns a similar distal or 

 marginal position of the sporangia is a feature not readily yielded up to 

 the advantage of better protection : it persists in the marginal sporangia or 

 sori so prevalent among the earliest of them. 



By the effective nutritional scheme of the sporophyte the Ferns have often 

 developed to large size, and have provided for a large output of spores, 

 which with few exceptions are all alike, homosporous. The success of their 

 propagation depends upon multiplying chances rather than upon that refine- 

 ment of detail which comes with heterospory. Those Ferns which adopted 

 the latter device never established themselves as real competitors with the 

 Seed-bearing Plants. In point of fact the Filicales present to us a singularly 



