468 



General Botany 



the wind. The development of light-weight spores is a fifth im- 

 portant characteristic of plants fitted to the land environment. 



Fig. 285. Figures showing the life history of a liverwort {Pallamcinia) : A, B, archegonial 

 and antheridial thalli; C, cross-section of thallus, showing antheridium ; D, cross-section 

 of thallus, showing archegonium ; E, F, G, stages in development of archegonium; 

 H, embryonic sporophyte within the greatly enlarged archegonium wall (calyptra) ; I, mature 

 sporophyte with spores. 



LIVERWORTS 



The body of many liverworts is fiat and leaf -like, and is called 

 a thallus (plural, thalli). It may be from one to several cell layers 

 in thickness. Growth takes place by repeated divisions of a 

 single cell at the tip. The thalli branch at intervals by forking. 

 Liverworts do not stand erect, but usually have their thalli in 

 close contact with the substrata on which they grow. In most 

 forms the thallus is a continuous plate of cells, but some forms have 

 prostrate stems with small leaves on either side. Even the 

 thalloid forms like Marchantia have scales on the under surface. 

 All the forms have small, hair-like rhizoids that anchor the plant 

 and absorb water and minerals. They reproduce by spores, 

 produced either directly on the thallus or on special reproduc- 

 tive branches. In some liverworts there are produced also 

 special bodies called gemmce (singular, gemma), which propagate 

 the plants vegetatively. 



The 4000 species of liverworts are widely distributed but are 



