AN INTRODUCTION TO PLANT BIOLOGY 99 



of the combustible matter of all plants. These ten elements 

 which are always found in plants must also be in their food, 

 and in the next three sections we shall consider how plants 

 get the food which will furnish these necessary elements for 

 making plant structure. 



97. Food-Materials from the Soil. It is a matter of 

 common observation that the growth of plants is largely 

 influenced by the nature of the soil. Every farmer and 

 gardener learns through experience to distinguish between 

 barren and fertile soils, and that the addition of manures 

 and various chemical " fertilizers " increases the growth of 

 plants. The relation of plant growth to the materials 

 available in the soil may be well illustrated by the following 

 experiment. 



(D) Growing Plants without Soil. This may be done by ger- 

 minating seeds of oats, beans, peas, lupines, and other common 

 plants on moist sawdust, cotton, sand, crushed stone, or other 

 materials into which roots can penetrate, but which contain no plant 

 food. When the seeds are well germinated, begin to moisten the 

 sawdust or cotton daily with water in which has been dissolved some 

 chemical tablets containing the materials such as are found in 

 good garden soil; that is, the necessary elements ( 96). If the 

 roots are kept moist with such a solution of chemicals, some plants 

 will develop flowers and seeds. By trying various chemicals in 

 such experiments, it has been possible for botanists to prove that 

 only certain elements are necessary in the soil for plant growth. 

 Most of the elements are common in agricultural soils, but com- 

 mercial fertilizers rich in nitrogen, potassium, calcium, and phos- 

 phorus are needed on many farms. The tablets may be purchased 

 from the Agassiz Association, Sound Beach, Conn., for ten cents a 

 box, post paid. 



Other interesting experiments in the same line may be performed 

 by growing plants in different kinds of soil in pots, fertilizing the soil 

 with various kinds of plant foods and fertilizers sold for garden use 

 (see catalogues of seed-dealers). 



98. Food of Plants without Chlorophyll. We can better 

 understand how a bean plant or other green plant gets its 



