78 PLANT-LIFE ON LAND [OH. 



families of plants belonging to the Gymnosperms. 

 Such plants, as their name implies, show their primi- 

 tive character by bearing their seeds exposed, as in 

 the Pine or Yew, and not contained in a seed-vessel, 

 like the more advanced Flowering plants. It is in 

 the Cycads and in the Maiden-hair Tree (Ginkgo) that 

 fertilisation by motile spermatozoids has been found 

 to occur. In both cases the pollen-grain germinates 

 essentially as in other Flowering plants. But the 

 gametes which each produces, instead of being passive, 

 are motile, by means of cilia, in fluid extruded into 

 a hollow chamber at the apex of the ovule : and thus 

 access is gained to the egg (Figs. 16, 17). The con- 

 clusion which follows from such facts is unavoidable. 

 The condition of the Cycads and Ginkgo can only be 

 held to be vestigial ; moreover the motility of their 

 gametes is of little practical use. The gametes of the 

 higher Flowering plants must then be regarded as 

 spermatozoids which have given up this unnecessary 

 motility, and become passive. Thereby they have 

 parted with the last indication of their pristine mode 

 of fertilisation by means of gametes freely swimming 

 in water, and show full accommodation to life upon 

 Land. But the Cycads and Ginkgo still show traces of 

 their amphibial origin. The rest of the Seed-plants, 

 however, now constitute a true Flora of the Land, 

 characterised as such by their complete independence 

 of external fluid water as a medium for fertilisation. 



