1 6 BIOLOGICAL LECTURES. 



of the heterosporous Pteridophytes, and in the lower forms, like 

 the coniferous trees, archegonia are produced. The embryo 

 sporophyte is formed from the fertilized egg-cell, as in the ferns, 

 and is surrounded by the tissue of the gametophyte, here called 

 the " endosperm." During the growth of the embryo the outer 

 tissues of the sporangium become hard, and finally the sporan- 

 gium with the enclosed spore and gametophyte falls away as a 

 seed. 



The permanent retention of the macrospore within the spo- 

 rangium necessitates a somewhat different mode of fertilization, 

 and we find, therefore, that the pollen spore, instead of devel- 

 oping an antheridium and motile spermatoids, produces a long 

 tube into which the sperm-cells pass. The pollen tube grows 

 through the tissues overlying the apex of the embryo sac, much 

 as a parasitic fungus penetrates the tissues of the host plant. 

 When it reaches the archegonium the sperm-cells are dis- 

 charged, and one of them passes directly into the egg-cell. 



One of the most important discoveries of recent years is the 

 presence of large ciliated spermatozoids in the lowest types of 

 seed-bearing plants ; e.g., Zamia, Cycas. In these the pollen 

 tube becomes much distended with water, and finally bursts, 

 discharging the large spermatozoids with the fluid into a space 

 above the archegonium, which the spermatozoids enter just as 

 they do in the ferns. This discovery removes the last barrier 

 separating the ferns from the flowering plants, and there can 

 be no further doubt as to the close relationship of these 

 groups. 



The Gymnosperms — Cycads, Coniferae, etc. — are undoubt- 

 edly much nearer the ferns than they are to the ordinary flow- 

 ering plants, or Angiosperms. It is usually taken for granted 

 that the latter originated from the Gymnosperms, but it is quite 

 as likely that they constitute an entirely independent develop- 

 mental line originating directly from the ferns, which has proved 

 itself better fitted on the whole to modern conditions than are 

 the simpler and more ancient Gymnosperms, which they have- 

 largely superseded. 



Although the sporophyte in some of the Gymnosperms, such 

 as the giant conifers, is often of gigantic size, it never shows as 



