LIVERWORTS 345 



shown in the capsule (sporogonium). This is a specialized 

 structure developed from the egg, which is very different in form 

 from the thallus, and its function is the development of spores 

 which produce new thallus plants and serve to propagate the 

 liverworts (a few are propagated also asexually by brood buds as 

 in Marchantia). Among the liverworts the capsule shows con- 

 siderable progress in development, from the simple spherical 

 body in Riccia, with a single layer of wall cells surrounding the 

 spore-bearing tissue, and the short-stalked capsule in Marchantia, 

 to that of the foliose liverworts, where the capsule is long-stalked 

 and is more highly specialized, splitting at maturity into four 

 valves (except in a few forms). But the highest specialization is 

 reached in Anthoceros where the capsule has a definite growing 

 area, the wall is provided with stomates and chlorophyll, and 

 the amount of spore- bearing tissue is very much less in propor- 

 tion to the sterile portion. These characters of the capsule 

 are of greater importance in showing relationship among the 

 liverworts than the form of the thallus. The marchantia and 

 riccia forms are all thallus plants with a comparatively simple 

 capsule. They belong to the Order Marchantiales. The foliose 

 liverworts have a more highly specialized capsule (the capsule 

 four-valved). They belong to the Order Jungermanniales, but 

 some members of this order also are thallose liverworts. The 

 highest liverworts are represented by Anthoceros with its highly 

 specialized capsule. They belong to the Order Anthocerotales. 

 They are also thallose liverworts. 



497. Alternation of generations. It is now time to note 

 an important biological principle in the life history and devel- 

 opment of plants, viz., what is usually called " alternation of 

 generations." The thallus with its sexual organs is the first gen- 

 eration, while the capsule is the second generation. The cap- 

 sule is developed from the fertilized egg on the thallus, and the 

 thallus is developed from the spore borne in the capsule. There 

 is thus an alternation of the thallus and capsule, or an alternation 

 of these generations. This does not mean that this alternation is 

 strictly carried out. The thallus is usually perennial and lives 

 for many years, developing capsules perhaps each year. 



