THE VARIOUS FORINIS OF PLANTS 



155 



Prothalium of a common tern (aspidium) : 

 A, under surface, showing rhizoids, )A, 

 antheridia, an, and archegonia, ar; B, 

 under surface of an older gametophytc, 

 showing rhizoids, rh, and young spo- 

 rophyte, witliroot, w, and leaf, fc. (From 

 Coulter, Plant Structures.) 



Fertilization. — The spprin cells swim to tho ogg colls in water (rain or 

 dew), being attracted Id the mouth of the flask-shaped archegonium by 

 an acid secretion which is poured out by the cells forming the neck of the 

 flask. Fertilization is essentially 

 the same process that has been 

 described for the flowering plants, 

 the sperm cell uniting with the 

 egg cell to form a single cell, the 

 fertilized egg. 



Sporophyte and Gametophyte. 

 — The direct result of fertilization 

 is the growth of the egg cell by re- 

 peated division to form a little 

 fern plant. Later the young plant 

 strikes root, the prothallus dies 

 away, and we have a fern plant 

 which will later in the season pro- 

 duce asexual spores. The leafy 

 fern plant, because it produces 

 asexual spores, is called the sporo- 

 phyte. The prothallus, which forms 

 the eggs and sperms, both of which 

 are known as gametes, or sex cells, is called the gametophyte. 



Alternation of Generations. — The fern plant like the moss also passes 

 through two entirely different stages, or generations. The spore ger- 

 minates to form a gametophyte, or 

 sexual generation. This sexual 

 generation in turn produces an 

 asexual generation, or sporophyte. 

 The alternation, in the life history of 

 a plant or animal, of a sexual stage 

 with an asexual stage is called an 

 alternation of generations. 



General Characters of the Fem- 

 like Plants. — These plants pass 

 through an alternation of genera- 

 tions; they ha^'e a distinct root, 

 stem, and leaves ; and the stem 

 possesses conducting tubes or fibro- 

 vascular bundles ; these are the 

 distinguishing marks of the ferns and their allies. Fern plants show a 

 great diversity in form and size. They vary from the great tree ferns of 

 the tropics, some of which are thirty to forty feet in height, to tiny forms 

 of almost microscopic size. The leaves of the ferns are among the most 

 complex in form of any that we know. 



1, the archegonium with egg (e) and canal 

 (c) ; B, antheridium ; C, antherozoid, 

 very highly magnified. — Strasburger. 



