100 



MORPHOLOGY 



produce spores, is said to be sterile. This sterile tissue in Marchantia^ 

 which was sporogenous tissue in the Ricciaceae, forms a region of the 

 sporophyte quite distinct from the spore-producing region, so that in the 

 mature sporophyte three regions are recognized: 

 (i) the capsule, containing the spores; (2) the 

 seta, a short stalk beneath the capsule, which 

 elongates rapidly as the capsule matures; and 

 (3) the foot, a spreading structure in which the 

 seta ends, and which anchors the sporophyte in 

 the gametophyte and also acts as an absorbing 

 organ (fig. 230). 



The spore production of Marchantia is further 

 diminished by the fact that not all the potentially 

 sporogenous cells produce spores. Some of them 

 become converted into curious elongated, fiber-like 

 cells with spiral thickening (elaters), which by their 

 hygroscopic movements assist in loosening up the 

 spore mass in connection with dehiscence and 

 scattering. The usual calyptra is formed about 

 the developing sporophyte by the venter of the 

 archegonium, but at maturity the capsule breaks 

 through this by the rapid elongation of the seta, 

 (embedded in tissue of After its escape from the calyptra the capsule 



gametophyte). , . j j- u 



breaks irregularly and discharges its spores. 



Conclusions. The Marchantiales are characterized by a distinct 

 and strong differentiation of the tissues of the gametophyte, leading to 

 the highly complex thallus of Marchantia, with its specialized mechanism 

 for photosynthesis. There is observable in the group also a distinct 

 tendency in the sporophyte toward the sterilization cf the potentially 

 sporogenous tissue into the sterile tissue developing the seta and foot. 

 This means that seta and foot are derived historically from sporogenous 

 tissue. The development of the seta, moreover, is associated with the 

 dispersal of spores, its rapid elongation freeing the capsule from the 

 calyptra. The further sterilization of potentially sporogenous tissue in 

 the production of elaters is another feature of the group, and is also 

 associated with spore dispersal. On the whole, the chief distinction of 

 Marchantiales as compared with the other groups of liverworts is the 

 differentiation of the tissues of the gametophyte, which has reached ?ts 

 extreme expression in Marchantia. 



FIG. 230. 

 chantia : sporophyte, 

 showing capsule (con- 

 taining spores and ela- 

 ters), seta, and foot 



