ORGANIC EVOLUTION 123 



During the fall and winter the disc is low and inconspicu- 

 ous, being contained in a circular pit in the top of the thal- 

 lus, with only the conic cap projecting. It is surrounded by 

 a circular cleft which admits of access to the archegonia 

 when these are mature. The sperms are free swimming 

 cells of strictly aquatic habit. They can make their egress 

 from the antheridia and swim about to find the egg cells 

 only when rain or dew has supplied sufficient water for the 

 purpose. 



In early spring the stalk of the archegonial disc elongates 

 enormously and pushes the cap up in the air to a height of 

 two or three inches (fig. 71 e). This lengthening is not 

 brought about by the production of new cells, but by the 

 further development of those already present. In the cen- 

 ter of the transparent stalk may be seen an axial bundle of 

 cells that have become extraordinarily lengthened, and 

 that form when fully developed and hollow, a bundle of 

 capillary tubes, which facilitate the transport of food 

 materials from the thallus below to the maturing spores 

 above. These capillary cells are the most specialized cells 

 in the body of the liverwort. 



Development. From the fertilized egg cell there devel- 

 ops, not another thallus like the one that produced it, but a 

 plant body of a very different sort, (fig. 72 M), called a 

 sporophyte (or sporogonium) . This develops as follows: 

 The egg cell divides repeatedly, forming a mass of cells that 

 distends the walls of the archegonium. This cell mass then 

 differentiates gradually into the three parts of the sporo- 

 phyte, foot, stalk and sporangium. The sporophyte 

 develops at the expense of the archegoniophore ; its foot 

 remains immersed in the tissues surrounding the base of the 

 archegonium, and serves as the food-absorbing organ. The 

 stalk gradually elongates and pushes the sporangium down- 

 ward toward the outer world. The sporangium develops a 



