

A MOSS 93 



(Fig. 47, C). This alga-like plant is called a protonema. 

 Some short green branches of the protonema grow upright, 

 instead of creeping as the longer branches do. Other 

 branches, losing their chlorophyl, push downward into the 

 soil or into the crevices of the substance on which the 

 protonema is growing ; these colorless or brownish branches 

 are rhizoids. The protonema grows and branches in this 

 way for some time ; but sooner or later a bud appears which 

 develops, not into a thread-like branch, but into a rounded 

 group of cells ; and this group of cells grows into an upright 

 branch or stem that bears leaves and rhizoids. This upright 

 branch is in fact exactly like the plant with which we began 

 the study of the life of the moss. Since a single protonema 

 may bear several of these leafy branches, it might be more 

 accurate to consider the protonema together with all the 

 upright branches growing from it as a single plant ; but as 

 the protonema dies sooner or later, each leafy branch comes 

 to be actually a separate plant, and so it is most convenient 

 to speak of it in that way. 



122. The Two Generations of the Moss. The whole life 

 of the moss divides itself naturally into two parts. The first, 

 beginning with the spore, includes the protonema and the 

 leafy branches or plants which grow from it ; in this part of 

 the life cycle the gametes (egg and antherozoid) are formed. 

 The second part of the history consists of the small plant 

 which grows from the zygote, and which in turn bears spores. 

 As we have seen, this spore-bearing plant is parasitic upon 

 the larger one which bore the egg, but it is none the less a 

 distinct plant. These two divisions of the life of the moss 

 are two distinct generations; the longer-lived one, in which 

 gametes are produced, is the sexual generation; the shorter- 

 lived one, in which spores are produced, is the asexual genera- 

 tion. In tracing the history of the moss, we must follow 

 through these two generations in order to return to the point 

 at which we started ; so this history may be represented, as 



