726 TRANSACTIONS OF SECTION K. 



Though remote from the series that have culminated in seed-plants, the 

 Bryophytes are a group offering many an instructive parallel with the main series 

 of plants ; certainly these forms have remained too long a thing apart. Haberlandt 

 and Goebel have shown us — to name no others — -how happy is the hunting-ground 

 which the Bryophytes provide. Further work is still required, directed more 

 especially to certain important points in the life-history. 



With the regular vascular cryptogams the relations between the stages are of 

 course different. Here we find large complex sporophytes holding the ground, 

 but hampered by the ever-recurring necessity of dependence upon outside water 

 for the performance of the reproductive process. 



The land problem was solved on ingenious lines. The differentiation of gameto- 

 phytes which accompanied heterospory rendered possible the retention of the 

 larger spore and female prothallus. Thus retained aloft, the drawback of the 

 double existence is overcome and the advantages of the elaborated sporophyte 

 more fully realised. The water conditions are brought directly under the plant's 

 control through the device of the pollen-chamber, and the way paved for the ideal 

 seed with siphonogamy. 



All the elements of the seed were present before, but combined compactly in 

 this new way we recognise what is virtually a fresh stage intercalated in the 

 life-history. Further elaboration came bit by bit as the possibilities were succes- 

 sively realised. With the evolution of the seed, the plant rose at a bound to 

 a higher plane, and this structure in its perfected form has become the very focus 

 of tlie plant's existence. 



The case of Cycns and Ginkgo with motile sperms affords an extreme demon- 

 stration of the inertia of heredity, the persistence in living seed-plants of the 

 original aquatic flagellate type. 



Obsolete as they are and faced with extinction, these survivors from the 

 middle epoch of the world's history still bold their ground in a few scattered 

 localities. In this connection we shall listen with interest to Professor Pearson's 

 account of the Ence>)7ialartos-siiT\i\) of South Africa which is to occupy us during 

 the course of the present sitting of the Section. 



How the sperms became replaced ultimately by the passive cells of the pollen- 

 tube we have no knowledge. 



If the conjecture be well founded that the change came late rather than early, 

 then the conservatism of the spermophytic line in this respect stands in marked 

 contrast to the adaptability that is so characteristic of another phylum of aerial 

 plants. The ready evolution of siphonogamy in the form of fertilising tubes, so 

 common in the Fungi, perhaps finds its explanation in the close filiation of this 

 group with primitive and plastic forms. The fertilising tube may reasonably be 

 regarded as a special case of a general susceptibility to chemiotactic stimuli which 

 distinguished the whole hyphal complex of the group from very early times. In 

 the case of the .''permophyte, on the other hand, the motile spermatozoid seems 

 to have persisted through a long and complicated ancestral history, so that its 

 eliminacion may have been less easy of achievement. 



The seed, once evolved, became the centre of a host of accessory organs, 

 constituting what we know collectively as the fruit and flower. By these it has 

 been robbed, as we shall see, of many of its pristine functions, whilst at the same 

 time it has undergone marked structural reduction. In the highly elaborated 

 Angiosperm more especially we find an almost stereotyped uniformity in seed- 

 structure contrasting with an infinite diversity in the outward floral husk. 



In attempting a sketch of the origin of the seed one has to admit at the outset 

 that recent discoveries bring us no nearer to its prototype than we were a decade 

 ago. For the seeds of the Pteridosperms are advanced structures recalling quite 

 vividly the type long familiar in living Cycads. It would be overstating the case 

 to say they have nothing primitive about them, but there is a long chapter in 

 evolution to be deciphered before we can connect, say, the seed of Lyginodendron 

 with the sporangium of any Fern at present known to us. 



The great interest of the recent correlation of seeds with Coal^Measure plants 

 lies less in the structure of these correlated seeds than in the very "extensive series 



