GROUPS OF PLANTS. 



51 



The protonema sooner or later dies off in most plants, but in 

 other cases it persists, forming a conspicuous portion 'of the 

 gametophyte. 



HEPATIC^. 



General Structure. The Hepaticae or Liverworts (Fig. 

 29) are usually found in moist situations. The protonema 

 formed on germination of a spore is filiform, and the plant 

 body which develops from it consists of a flat, dichotomously- 



FiG. 30. Transverse section through the thallus of Marchantia polymorpha. A, 

 middle portion with scales (b) and rhizoids (h) on the under side; E, margin of the thallus 

 more highly magnified, showing colorless rcticuktcly thicliened parenchyma (p), epidermis 

 of the upper side (o), cells containing chlorophyll (chl), air pore (sp), lower epidermis (u). 

 After Goebel. 



branching thallus, or it may in some of the higher forms differ- 

 entiate into a leafy branch as in the leafy liverworts. The thallus, 

 owing to its position, has an upper and an under surface which are 

 somewhat different, as in Marchantia (Fig. 29), hence it is said 

 to be DORSiVENTRAL. From the lower colorless surface unicellular 

 rhizoids arise (Fig. 30, h). The upper surface consists of several 

 layers of cells containing chlorophyll which give the green color 

 to the plant. 



