SUMMARY OF THE BRYOPHYTES 303 



in some situations, d,o not constitute so effective a plant body as 

 a leafy stem with an erect habit, which secures a much greater 

 exposure to air and light. Accordingly the appearance of leafy 

 stems marked a great advance over the thallus structure. This 

 new form of bryophyte plant body reached its highest develop- 

 ment when the stem became erect with the leaves arranged 

 spirally, as in the mosses, so as to give a radial symmetry. 



It is quite safe to say that the adoption of the land habit 

 was the chief cause of the rapid advance of the bryophytes over 

 the algse. The advance in vegetative structure is generally most 

 marked in the gametophyte phase of the life history, although 

 the sporophytes of such types as Anthoceros and certain mosses 

 are clearly higher than the gametophytes. It may be noted in 

 this connection that the next great forward step in the evolu- 

 tion of plants came in the fern group, or pteridophytes, when 

 the sporophyte generation adopted the land habit and became 

 independent of the gametophyte. However, this subject will be 

 taken up in the next chapter. 



SUMMARY OF THE BRYOPHYTES AND 

 THALLOPHYTES 



299. Bryophytes and thallophytes compared. It is possible 

 at this point to make clear the fundamental reasons for the 

 separation of the spore plants, so far studied, into the two great 

 divisions of the plant kingdom called the Thallophyta and Bryo- 

 phyta. It will be seen that the bryophytes have a set of very 

 clearly denned characters, while the thallophytes are distin- 

 guished largely by the absence of these. 



300. Summary of the bryophytes. The sexual organs are 

 many-celled structures differentiated into f emaje organs (arche- 

 gonia) and male organs (antheridia). The fertilized egg develops 

 at once into an asexual generation, or sporophyte, which pro- 

 duces asexual spores in groups of four, or tetrads, within cer- 

 tain cells called spore mother cells. The sporophyte, often called 



