ORGANIC EVOLUTION 127 



row of teeth attached at the margin, with free tips that 

 meet at the center of the columella. These regulate the 

 dispersal of the spores, by lifting (as if by hinges at their 

 bases) when dry , and exposing the top of the spore cavity, 

 allowing the spores to be shaken or to fall out, and closing 

 down by hygroscopic movement when wet. 



Perhaps the most significant feature is the appearance of 

 a loose layer of chlorophyl bearing parenchyma in the outer 

 wall of the sporangium, and the development in the epider- 

 mis covering the base of it, of a few breathing pores (sto- 

 mates) of a new type. This type is the prevailing one 

 in the following groups of plants. Thus the sporophyte is 

 able to gather some carbon for itself, though dependent on 

 the gametophyte for other elements of its food. 



We have already seen that, as compared with the algae, 

 the bryophytes have attained to a higher grade of bodily 

 structure, and have acquired an organization that is 

 directly related to terrestrial life. The new features of their 

 life history, also, are related to this changed environment. 

 They brought with them from the water and have one and 

 all retained a strictly aquatic mode of fertilization. All 

 have free-swimming sperm cells : and the distance between 

 the antheridia and the archegonia must be covered by water 

 at the time the sex cells ripen, else fertilization cannot take 

 place. But spores do not require to be fertilized : and the 

 introduction of the spore-producing generation seems to 

 have been the means availed of for securing the production 

 and distribution of a host of new individuals while avoiding 

 the exigencies of fertilization under the changed conditions. 



Study 17. An examination of bryophyte characters. 



Materials needed : Fresh green specimens of Conocepha- 

 lus or other thalloid liverwort ; either fresh or preserved 

 specimens bearing antheridia and archegonia, older speci- 

 mens bearing sporophytes; also, sets of prepared slides. 



