THE BRYOPIIYTA 121 



go back to the fertilised ovum, and see how the fruit, 

 which here represents the asexual generation, is 

 developed. 



b. Development 



After fertilisation, the ovum first divides by a trans- 

 verse wall, but of the two cells thus produced the lower 

 takes no part in the further development. Capsule and 

 seta are both entirely derived from the upper cell, which 

 next divides by a vertical and then by a transverse 

 wall, the latter marking the boundary between capsule 

 and seta. Cell-walls parallel to the external surface 

 now appear in both parts. In the young capsule these 

 divisions separate the external layer from the archesporium, 

 which constitutes the whole of the internal group. In- 

 numerable cell-divisions take place, keeping pace with 

 the growth both in seta and capsule, until a stage is 

 reached like that shown in longitudinal section in Fig 

 55. The seta at its lower end develops a conical foot, 

 with a rim projecting upwards, firmly fixing the seta in 

 the thallus, with which, however, there is never any 

 real continuity of tissue. 



The seta itself consists throughout of very uniform 

 short-celled parenchyma. The capsule has a wall three 

 or more cells in thickness ; from the internal mass of 

 cells the sporogenous tissue is produced, but the whole of 

 this is not used up to form the mother-cells of spores. 

 A certain number of the cells grow in length, while 

 remaining narrow, and ultimately form long tubular 

 structures with, a double spiral thickening. These curious 

 elements very characteristic of the Liverworts are 

 called the elaters. When young, the elaters no doubt 

 serve to convey food-substance from the seta to the 



