DEVELOPMENT OF PLANTS 195 



number of chromosomes which appear in two-spore bearing 

 plants, i. e., the minute plant in the cystocarp and the tetraspore- 

 bearing plant. The distinction between the sexual and asexual 

 generation is emphasized at this point because we see in the algae 

 how it gradually became more and more conspicuous, and in the 

 mosses and succeeding groups it will be noticed that the relation 

 of these two generations is intimately associated with the evolu- 

 tion of the higher types of plant life. 



Subdivision 5. Eumycetes or Fungi 



76. The Nature of Fungi. — The Fungi are the largest group 

 of the Thallophyta and include such familiar forms as moulds, 

 mildews, toadstools and mushrooms. The absence of chloro- 

 phyll is the most striking feature of these plants. They are 

 unable therefore to form sugar, starch and other foods from the 

 elements of the soil and air and must obtain them already manu- 

 factured. Consequently, they are either saprophytes living upon 

 decaying organic matter, or parasites preying upon living organ- 

 isms. The majority of fungi procure their food from decaying 

 plant and animal matter, and in this relation many of them are 

 of the same economic importance as the saprophytic bacteria. 

 Other forms exist as parasites upon living plants and animals. 

 In this relation fungi cause almost incalculable loss annually to 

 the country and frequently crops over large areas are ruined by 

 their depredations. The parasitic forms gain access to the host 

 through a wound, stoma or the delicate tissues of seedlings or 

 young parts of the plant, and in other cases the fungus forms a 

 ferment that dissolves the cell walls and thus opens the way for 

 the entrance of the pest. The cultivation of plants has doubtless 

 weakened their resistance in many cases to the attack of the para- 

 sites, and like ourselves the plant may inherit a weaker constitu- 

 tion that is more subject to disease. Every state now employs 

 experts to study the diseases caused by these fungi and to devise 

 means for killing them and to develop more resistant plants. 



The fungi are of very simple constructive because they live 

 upon foods already manufactured for them. There is no longer 



