CHAPTER II. 



THE REQUIREMENTS OF PLANTS. 



FOR a true understanding of our subject it is necessary at the outset to 

 realise the conditions and factors influencing the growth of plants. We 

 have to look upon the plant as a synthetic agent and accumulator of 

 energy, taking up simple substances like carbon dioxide, water, nitrates, 

 phosphates, potassium salts, etc., and manufacturing complex sugars, 

 starch, cellulose, proteins, nucleo-proteins, essential oils, colouring 

 matters and a host of other substances. The natural object of the 

 processes is to produce seeds containing the embryo and a supply 

 of food for the young plant to draw upon till such time as it can syn- 

 thesise its own food. In agriculture, .however, the stored up food 

 material is taken at whatever stage is convenient and constitutes the 

 food and energy supply of animals and of men. 



As the processes of the plant are endothermic the energy of the 

 sun's rays is indispensable to them. The transforming agent is chloro- 

 phyll, the ordinary green colouring matter of the leaf. Since the 

 reactions have all to go at ordinary temperatures catalysts are neces- 

 sary to accelerate changes that would otherwise be very slow ; these 

 are supplied by the protoplasm and the numerous enzymes. The 

 whole cycle of changes collectively spoken of as plant growth repre- 

 sents the net gain from two opposite processes, (i) a constructive 

 process of at least three stages : synthesis of complex material, trans- 

 location of the synthesised food to centres of growth, and building up 

 of the food into plant tissues or reserves, (2) a respiratory process 

 whereby carbohydrate material is broken down and carbon dioxide 

 evolved. The synthesis is of two types : photosynthesis, in which 

 sugar is produced ; and another, not specifically named, giving rise to 

 protein. Photosynthesis, as its name implies, takes place only in light 

 and is restricted to the chlorophyll cells. The initial substances are 

 carbon dioxide and water ; in a very short time the apparent end 

 products, starch and oxygen (equal in volume to the carbon dioxide), 

 appear. It is shown, however, by the researches of Brown and others 

 that the real end product is cane sugar, starch only being formed when 



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