10 PRACTICAL BOTANY. 



archegonium, while in all the other groups it is essentially 

 an archegonium. The three higher groups are, therefore, 

 sometimes grouped together as archegoniates. The Thal- 

 lophytes, also, in many cases reproduce themselves asexu- 

 ally by cell division, and by the formation of sporelike 

 cells called gonidia, which are distinguished from ordinary 

 spores by the fact that they do not enter into the alterna- 

 tion of generations. Ordinary spores are borne by spo- 

 rophytes, and develop into gametophytes. Gonidia are 

 borne by gametophytes, and develop into gametophytes. 



The Bryophytes. Here also the ordinary form of the 

 plant is the gametophyte. The alternation of genera- 

 tions is well marked. The ordinary Moss plants produce 

 male gametes, antherozoids or spermatozoids, and- female 

 gametes, oo spheres. These unite and produce oo spores, 

 which develop in the archegonium into sporophytes. The 

 sporophyte produces spores, which grow first into small, 

 flat cell aggregates known as protonemce. The protonema 

 throws out lateral, or in some cases terminal, buds, which 

 develop into gametophytes or ordinary Moss plants. There 

 is no reproduction by the formation of gonidia, as in the 

 Thallophytes, but there is a vegetative reproduction by 

 gemmce, which form on various parts of the gametophyte. 

 A differentiation into root, stem, and leaf appears in a 

 rudimentary form. There are no true roots, but uni- 

 cellular or multicellular filaments known as rhizoids per- 

 form the functions of roots. Rudimentary fibrovascular 

 bundles also make their appearance. 



The Pteridophytes. Here, in contrast with the Thallo- 

 phytes and Bryophytes, the ordinary form of the plant is 

 the sporophyte. The alternation of generations is sharp. 

 The spore develops into a flat aggregate of cells called the 

 prothallium, which resembles in function the protonema of 

 the Bryophytes. The prothallium is the gametophyte. 

 It bears in specially differentiated cells antherozoids or 



