DEVELOPMENT OF PLANTS 



275 



tions of a lobing in the thallus which becomes more pronounced 

 in some forms and leads by gradual gradations to genera in which 

 the lobes appear as distinct leafy organs arranged in two rows 

 upon a stem-like axis (Fig. 197). Usually a third row of rudi- 





Fig. 197. One of the leafy Jungermaniales, Porella: A, branch of the 

 plant bearing several sporophytes. B, under surface of a branch, showing 

 the lobing of the leaves and a row of minute scale-like leaves. C, portion 

 of a branch bearing an archegonium surrounded by cup-like perianth with 

 minute involucrate leaves at: base. D, branch with cone-like antheridial 

 branchlets. At the left a single leaf is shown with globular antheridium 

 in its axis. E, section of a branch similar to C. The archegonium is seen 

 surrounded by the perianth and below the involucrate leaves. The sporo- 

 phyte is nearly mature and ready to elongate. It consists of a round cap- 

 sule containing elaters and spore mother cells that are dividing to form four 

 spores each. Below the capsule is the stalk or seta which ends in a foot 

 buried in the tissues of the branch. At the right is an unfertilized arche- 

 gonium that shows the original position and size of this organ. — H. O. Hanson. 



