CHAPTER XI 



THE BRYOPHYTA : HEPATICAE, THE LIVERWORTS 



With the exception of the higher Fungi, which lead a very speciaHzed Hfe, 

 the Thallophyta and Charophyta are essentially aquatic plants. Even in the 

 few instances where a terrestrial habitat has been adopted the reproductive 

 bodies rely upon water for their distribution. 



When we turn to consider the Bryophyta we come to a group of plants 

 which are essentially land-inhabiting organisms. Although they still rely 

 to some extent upon water for the movement of their gametes, in their 

 vegetative structure they have adapted themselves to a terrestrial life. They 

 have been likened to the Amphibia of the Animal Kingdom, and this simile 

 is extremely apt in so far as their mode of living is concerned. 



If the view is held that plant life originated in the water, the migration 

 from the water to the land must have been a very gradual one. Due possibly 

 to increased competition in the water, plants gradually adapted themselves 

 to colonizing the damp soil at the edge of the water, whence they migrated 

 further and further on to the land, relying on the rain and dew to supply 

 them with the water necessarv^ for life. We can picture this gradual migration, 

 but it must be confessed that we have very little direct evidence for it. One 

 of the largest gaps in the evolutionary sequence of plants is that which 

 separates the Thallophyta from the Bryophyta. Since the transitional types 

 would be necessarily small delicate organisms, it is hardly to be expected 

 that any of their remains would be handed down to us in fossil form. On 

 the other hand, we might have expected to find among the present-day 

 forms some types which could be termed " living fossils," in which the 

 primitive characters of the intermediate or " bridging " species were still 

 retained. This is not the case, and we are left to conjecture what these 

 intermediate types might have looked like. 



Turning to the consideration of the Bryophyta as a whole, they may be 

 described as plants in which the body is of fairly simple structure, develop- 

 ing, at least in the simpler members, a flat thallus resembling in vegetative 

 structure that of some of the Thallophyta but showing a marked advance 

 in the method of reproduction. The most important advance has been the 

 retention of the female gamete within a female organ with a cellular wall and 

 neck. This gamete, the oosphere, is not set free as in such Algae as Dictyota 

 or Fucus, but remains enclosed in the female organ, the archegonium, and 

 is fertilized by an actively motile antherozoid. Moreover, the embryo 

 which develops as a result of fertilization is not liberated but remains attached 

 to the parent plant, being nourished by it through an attaching organ or 

 foot. This embryo differs from the parent in that it is diploid and that it 



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