FERTILISATION 281 



the thallus is covered with a film of moisture. In forms like 

 Marchantia, the stalk bearing the lobed archegonial disc does 

 not lengthen till after fertilisation, so that the necks of the 

 archegonia remain in close contact with the moist upper surface 

 of the thallus. In the same way the branches bearing the 

 archegonial groups of Mosses are at first usually short and 

 adjacent to the soil, so that they are readily covered with moisture 

 during rain. 



After escaping from the antheridia, the spermatozoids, swim- 

 ming in the surface films of moisture, are chemotactically 

 attracted to the open necks of the archegonia by substances 



or 



FIG. 154. A, Longitudinal section through the apex of the thallus of Pellia, 

 showing a group of archegonia (ar.) and the involucre (i.). r., rhizoid. 

 B, Single archegonium of Marchantia, on a larger scale. e. t egg ; 

 n., neck ; v., ventral canal cell. 



(cane-sugar in Mosses) contained in the mucilage formed by the 

 breaking down of the canal cells. The spermatozoid passes 

 down the neck-canal and enters the egg, after which nuclear 

 fusion occurs. 



The fertilised ovum secretes a cell-wall and, without any 

 resting-stage, commences to divide and grow into a new Moss 

 or Liverwort plant. This, however, differs fundamentally from 

 the parent, especially in being parasitic upon the latter, and at 

 maturity constitutes an organism (the sporogonium, Fig. 150 ; 

 Fig. 155, A) that is almost solely concerned in the production 

 and dispersal of spores. This simplicity of structure can possibly 

 be related to the parasitic habit, thus affording an interesting 



