80 A TEXT-BOOK OF BOTANY. 



microns, being sometimes larger. They occur in groups of four 

 in a mother-cell, and the spore-group is known as a tetrad, which 

 is characteristic for the Bryophytes and the higher groups of 

 plants. The spores therefore vary in shape from spherical tetra- 

 hedrons to more or less spherical bodies, depending upon the 

 degree of separation. The contents are rich in protoplasm and 

 oil (Fig. 45, F). The wall consists of two layers, the outer of 

 which is either yellowish or brown and is usually finely sculptured. 

 At the time of germination the outer wall is thrown off, and the 

 protonema develops (Fig. 45, G). The spores may germinate 

 almost immediately, or only after a considerable period. These 

 spores are asexual and each one is capable of giving rise to a 

 new plant. With the formation and dispersal of the spores the 

 work of this generation terminates, and this phase is called the 

 sporophyte or asexual generation, from the fact that it produces 

 spores. 



Having thus followed the stages of development in the life 

 history of a moss, we see that it is composed of the following 

 parts: (i) The alga-like protonema; (2) the leafy branch which 

 gives rise to an oospore (sexual spore), and (3) the sporogonium 

 which produces asexual spores. The leafy branch is sometimes 

 spoken of as the gametophore (gamete-bearer), and it and the 

 protonema together constitute the gametophyte or sexual gen- 

 eration, while the sporogonium represents the sporophyte or 

 asexual generation. 



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. 46) 

 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-b ranching 

 thallus, or it may in some of the higher forms differentiate into 

 a leafy branch, as in the leafy liverworts. The thallus, owing to 

 its position, has an upper and an under surface which are some- 

 what different, as in Marchantia (Fig. 46), hence it is said to be 



