THE VARIOUS FORMS OF PLANTS 



155 



Fertilization. The sperm cells swim to the egg cells in water (rain or 



dew), being attracted to 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 Fern- 

 like Plants. These plants pass 

 through an alternation of genera- 

 tions; they have a distinct root, 



Prothalium of a common fern (aspidium) : 

 A, under surface, showing rhizoids, rh, 

 antheridia, a/, and archegonia, ar ; B, 

 under surface of an older gametophyte, 

 showing rhizoids, rh, and young spo- 

 rophyte, with root, w, and leaf, 6. (From 

 Coulter, Plant Structures.) 



B A 



A, the archegonium with egg (e) and canal 

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

 very highly magnified. Strasburger. 



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. 



