156 THE VARIOUS FORMS OF PLANTS 



The Horsetails. These comprise a small group of plants, recognized 

 by their erect habit of growth, the leaves coming out in whorls on the stem. 

 In most forms the stem contains considerable silica. This gave to the 

 plant its former useful place in the household and its name of the scour- 

 ing rush. If you burn one of these plants very carefully on a tin plate over 

 a very hot fire, the delicate skeleton of silica may be seen. The horse- 

 tails, or Equisetums, were once a very important part of the earth's 

 vegetation. Before the coal fields were formed, the ancestors of these 

 plants flourished as trees. A large amount of the coal of this country is 

 undoubtedly formed from the trunks of the Equisetums of the Carbonif- 

 erous age. At present they are represented by a very few species, none 

 of which are over four or five feet in height. 



Club Mosses. Another relative of the fern is the club moss (lyco- 

 podium). It is familiar to us as a Christmas decoration under the name 

 of ground pine. It is chiefly of interest now as the representative of 

 another group of plants that flourished during the Carboniferous age. 



Economic Value of Ferns. It may be said that the ferns as a group 

 have formed a large part of the enormous deposits of almost pure carbon 

 that we call coal, from which we now derive the energy to run our many 

 engines. 



Sexual Reproduction in Flowering Plants. Flowering plants 

 reproduce their kind by the formation of seeds. As we know, the 

 flower produces in the ovary structures which are known as ovules. 

 In the interior of the ovule is found a clear protoplasmic area which 

 is called the embyro sac. In this area is a cell (the egg cell) which 

 is destined to form the future plant. In the pollen grain is found 

 another cell, the sperm. This cell, after the germination of the 

 pollen grain on the stigmatic surface of the flower, enters the ovule 

 in the pollen tube and unites with the egg cell. The fertilized egg 

 grows into the young plant within the seed, known as the embryo 

 (see page 37). 



This method of reproduction, called sexual reproduction, is 

 found in the spermatophytes, that is, all seed-producing plants. 



Botanists have shown that in the spermatophytes there exists 

 an alternation of generations as in the mosses and ferns. The 

 pollen grain is believed to contain the male gametophyte, while 

 within the embryo sac is found the female gametophyte. Most of 

 the life of the flowering plant is thus seen to be passed in the asexual 

 or sporophyte stage. Thus we see that all plants and all ani- 

 mals as well form the cells which compose their bodies by either 



