200 COLLEGE BOTANY 



The ash. content of the plant varies with the different ages of 

 the plant and in different parts of the plant. The development 

 of the fruit and seed is usually accompanied by the transloca- 

 tion of certain mineral elements and compounds. Phosphoric 

 acid migrates from the leaves and stems to the fruiting organs, 

 and other elements and compounds are transported from place 

 to place in varying amounts at different periodsi in the growth 

 of the plant. 



Origin of Minerals. — The soil is made up of ground rock and 

 humus. The rock is ground up by the action of water and frost 

 and many other natural agencies. In past ages the glaciers 

 which moved downward from the north and spread over a con- 

 siderable part of the eastern United States ground up the rock 

 and distributed great quantities of soil and debris. The effects 

 of these great ice movements are very evident to both the geol- 

 ogists and the botanists who make a study of plant distribution. 

 Burrowing animals of various kinds and sizes also exert a much 

 greater influence over the character of the soil than is generally 

 appreciated. They bring a certain amount of subsoil to the sur- 

 face and carry more or less organic material into their burrows 

 for food and bedding, which decays and gives rise to acids which 

 act on the surrounding soil. The biirrows also afford excellent 

 openings for the entrance of surface water. The ground materials 

 from different kinds of rock are mixed in varying proportions bv 

 water and other natural agencies, and give rise to many well- 

 known types of soil. It will be readily seen that these soils vary 

 in different parts of the earth, dependent on the character of the 

 material from which they are derived, and the action of water 

 and other agencies. These variations, together with other fac- 

 tors, such as humus, water and temperature, are the controlling 

 factors in plant distribution over the surface of the earth. 



