316 BOTANY: PRINCIPLES AND PROBLEMS 



bearing only anthcridia (Fig. 187) and some only archegonia 

 (Fig. 188). The sporophyte (Fig. 189) is more specialized than 

 that of Riccia for it includes not only a spore case but a short 

 stalk or seta the growth of which carries the spore case out and 

 away from the thallus. The base of this seta is enlarged into a 

 foot, anchoring the sporophyte in the tissue of the thallus and 

 absorbing food therefrom. Furthermore, not all of the central 

 portion of the sporogonium itself produces spores, for many of the 



Fig. 190. — Porella. Portion of thallus, showing the two rows of leaves. One of 

 the two capsules figured has opened and liberated its spores. 



cells grow instead into long, spirally thickened elements, the elaters, 

 which assist in loosening and scattering the spore mass at maturity. 

 The relative amount of sporophyte tissue which does not contrib- 

 ute directly to the production of spores, and is therefore called 

 sterile tissue, increases steadily as we trace the upward evolution 

 of the sporophyte. In the simplest case (among certain thallo- 

 phytes), the fertilized egg develops into a group of spores. In 

 Riccia, the only sterile tissue is the sporangium wall; in Marchan- 

 tia, the seta and foot are added; in the mosses still other regions 

 are "sterilized", and in the higher plants the spores them- 



