THE VARIOUS FORMS OF PLANTS 153 



If we were to plant the spores of the moss in damp sand, taking care to 

 keep the sand moist and warm, we might get them to grow. The 

 spore germinates into a threadlike structure, very tiny, and not at all 

 like the adult moss plant. This thread is called a protonema. 



Adult Moss Plants. The protonema soon develops rhizoids ; tiny 

 buds appear which in time form the adult moss plant. These adult plants 

 may grow only leaves, and become what are known as sterile plants ; or 

 they may develop into a plant that bears at the summit the little rosette 

 of leaves previously referred to. Within the rosette lie a number of tinv 

 organs which hold large numbers of sperm cells. Other moss plants not 

 so tall as the sperm-producing plants bear at the summit of the stem a 

 tuft of leaves which hide a number of small flask-shaped structures, each 

 of which contains a single egg cell. These plants form the sexual genera- 

 tion of the moss. This stage of the plant is called the gametophyte, because 

 it produces the gametes or sexual cells, eggs and sperms. After a sperm 

 cell has been transferred (usually by means of a drop of dew) to the egg 

 cell, a fusion of the two cells takes place. This, we remember, is the pro- 

 cess of fertilization. In the mosses the fertilization of the egg cell results 

 in the growth of that part of the plant which forms and bears the asexual 

 spores. 



Alternation of Generations. In the mosses we have what is known 

 as an alternation of generations. The leafy moss, bearing among its leaves 

 the organs producing sperms and eggs, antheridia and archegonia, gives 

 place to a stalk and capsule bearing the asexual spores. This spore-bear- 

 ing portion of the plant does not appear until after fertilization ; then it 

 grows directly out of that part of the plant which produces the egg cell. 

 In fact, if we make a microscopic examination of the egg-producing struc- 

 ture (the archegonium) directly after fertilization, we find that the sporo- 

 phyte is a direct outgrowth from the fertilized egg cell. Thus the sexual 

 stage alternates with the asexual stage in the life of the plant. 



Sporophyte a Parasite. One interesting fact comes out in connection 

 with this growth of the sporophyte. It has no green leaves and must 

 therefore obtain all its nourishment from the leafy moss plant, or game- 

 tophyte. The spore-bearing part of the plant is thus actually a parasite 

 upon the gametophyte. 



Ferns and their Allies 



The Ferns and their Allies. The fern plants include the true ferns, 

 the horsetails or scouring rushes, and the club mosses. The true ferns 

 are moisture-loving and shade-loving plants ; they play an important part 

 in the vegetation of the tropical forests. Many forms are found in the 

 temperate regions ; we even have some common ferns that remain green 

 all winter. Fossil ferns have been found in Greenland, thus showing that 

 at one time the climate at the north was milder than it now is. 



