THE SPERMATOPHYTA : GENERAL INTRODUCTION 641 



we contrast these advantages with the relatively imperfect provision made 

 for the safety and establishment of the young plant in lower groups we can 

 easily understand why the Seed Plants have become dominant in the world's 

 vegetation. 



Seed production in the Angiosperms is more efficient than in the majority 

 of Gymnosperms, in that fertilization in the higher group occurs at an earlier 

 stage of seed formation than in the lower group. In many Gymnosperms the 

 seed is completely developed and may in fact be actually shed from the tree 

 before fertilization, and should any accident prevent its accomplishment the 

 whole of the material involved is wasted. The angiospermic ovule, however, 

 is fertilized while still quite young, and unless it takes place there is no further 

 development. Thus the laying down of reser\-e food in the endosperm, 

 for example, is a post-fertilization process, in contrast to the prothallus 

 formation of the Gymnosperm which precedes fertilization. 



The whole of this development hinges on one cardinal fact : the reduction 

 of the free-living gametophyte, which, as the seat of sexual conjugation, is 

 inseparably linked to water}' conditions, and its enclosure within the body 

 of the sturdy sporophyte, which has no such limitations. With the gameto- 

 phyte thus enclosed, sexual conjugation takes place in an internal environment, 

 free from exterior influences, and the growth of the embryo from the fertilized 

 oosphere is likewise protected and ensured. 



Evolution of the Megaphyllous Strobilus 



We have seen that the Pteridosperms had no flowers. Their seeds were 

 borne, exposed and usually singly, on the fronds. In some types these fertile 

 fronds were leafy and were otherwise indistinguishable from the sterile 

 fronds, just as in Dryopteris at the present day, but in others the fertile fronds 

 had no lamina and were reduced to simple or branched stalks, which bore 

 either microsporangia or seeds, the latter sometimes protected bv a cup-like 

 cupule. It is immaterial for our present purposes to discuss whether these 

 specialized fronds or sporophylls were in fact evolved from leaves like the 

 foliage leaves, or whether the two had always been distinct structures. Either 

 view may be correct, but the important thing for us to notice is that in certain 

 types the production of sporophylls became restricted both in time and 

 space, so that they formed limited groups, either on the main axis or on side 

 shoots, where they were arranged in the same spiral succession as the foliage 

 leaves but closely aggregated to form strobili. This re-emergence of the 

 strobilus, a structure otherwise limited to the microphyllous Lycopodiales 

 and Equisetales, as an aggregation of megaphyllous seed-bearing sporophylls, 

 marks the beginning of the true Flowering Plants. 



It seems to have occurred first, so far as the megaphyllous Pteropsida 

 are concerned, in the Palaeozoic Cordaitales (Figs. 650 and 651). What 

 relation these plants bore to the Pteridosperms is uncertain, but it is probable 

 that both groups were descended from a common fern-like ancestrs', the 

 Cordaitales having evolved the strobilus and the Pteridosperms alone having 



