CHAPTER XVI 



THE SPERMATOPHYTA : GENERAL INTRODUCTION 

 WITH AN OUTLINE OF EVOLUTIONARY MOR- 

 PHOLOGY 



The Spermatophyta,^ commonly called the " Seed Plants," is the large 

 group of plants in which the chief organ of distribution is the seed. By this 

 term is implied a structure enclosing an embryo and protected by a special 

 covering, the testa. A store of reserve food material is also present, either 

 in the tissues of the embryo itself or in a special tissue, the endosperm, 

 which surrounds the embryo. 



The Spermatophyta have no doubt evolved from some lower group 

 among the Cryptogams, most probably from among the Pteropsida, but their 

 evolution took place at an early geological period, and their direct ancestors 

 have long been extinct. This evolutionary process depends essentially upon 

 the reduction of the gametophyte generation and its enclosure within the 

 tissues of the sporophyte, which thus became the only externally visible 

 generation in the life-cycle. We shall see later how this took place and 

 what were its biological consequences. It is noteworthy here, however, 

 that the tendency to a reduction of the gametophyte can be seen even among 

 Ferns with free-living prothalli, in which the precocious development of sex 

 organs on small prothalli is not uncommon. Goebel even records a case of 

 a Fern prothallus consisting only of one vegetative cell, which had produced 

 two antheridia. 



The reduction and enclosure of the gametophyte has only been rendered 

 biologically successful by the development of another new device, peculiar 

 to the seed plants, namely the transport of the male gametic cells to the 

 immediate neighbourhood of the female, in place of the free-swimming 

 antherozoid. This has involved two stages. The first is pollination, that 

 is, the provision of means whereby the microspores are conveyed to the 

 immediate neighbourhood of the megaspore and are there held while they 

 germinate. The second is the evolution of the pollen tube, as an outgrowth 

 from the microspore, which ensures the direct access of the male cells to the 

 female, without exposing them to the hazards of an external journey. The 

 importance of the latter factor can scarcely be overrated. It is the ultimate 

 answer to the problems of fertilization under land-living conditions and 

 serves to separate two great biological groups, the Zoidogamia, which 

 include all plants with free-swimming antherozoids, and the Siphonogamia, 



^ The term Spermatophyta is now generally used in preference to the older term 

 Phanerogam, which is based on a false antithesis to Cryptogam. The term Spermaphyta 

 is sometimes used but is not etymologically correct. 



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