PLANT PHYSIOLOGY 9 



essential for plant- life. The proportions in which 

 these elements are chemically combined form the 

 various substances which together make plant- 

 tissues — leaves, stems, roots, flowers, fruits, seeds, 

 etc. 



Plants contain moisture, oU, albuminoids, fibre, 

 carbohydrates, ash, etc. 



The ash (after burning a plant) consists of 

 certain mineral ingredients extracted from the soil 

 in a state of solution. 



Organic Matter. — -A certain quantity of organic 

 matter or humus is essential in a soil, for it causes 

 the disintegration or breaking up of the soil, and 

 renders it more friable. It yields nitrogenous 

 matters to trees, etc. — that is after bacterial oxida- 

 tion. An excess of organic matter, however, 

 renders a soil • " sour," and produces a harsh 

 growth, or destroys the trees or plants. 



Water and Mineral Matter. — There must 

 always be present a sufiiciency of water and 

 mineral matter in a soil for the growth of trees. 

 According to the law of minimum, a soU destitute 

 of any one of the mineral constituents may 

 become more or less barren ; since it is the 

 minimum of any one essential ingredient, and not 

 the maximum of others, which is the measure of 

 fertility. 



