THE GROWTH OF THE PLANT 89 



ence between the boiled and the unboiled seed: one 

 can germinate, the other cannot ; or, differently ex- 

 pressed, one is living, the other is dead. 



F. Heat as a Necessary Factor in Germination 



Heat and cold are relative and not absolute terms. 

 The seeds in our gardens do not germinate in the spring 

 until a certain degree of temperature is reached; 

 further, seeds of any kind kept in a warm greenhouse 

 germinate earlier than those of the same kind sown in 

 the open. 



Accurate experimental work on this point is beyond 

 the scope of this book. It can only be stated that 

 some degree of warmth is necessary for germination 

 and that the degree differs for different seeds. 



Plants that require a large amount of heat must grow 

 in tropical countries ; in colder countries are to be found 

 only those plants that can thrive at lower temperatures. 



In this country the limits of temperature between 

 which plants grow are a- few degrees above freezing- 

 point and about 50° C, but the plant's greatest activity 

 occurs between the temperature 25° C. and 30° C. 



The conditions under which a seed can germinate 

 have now been investigated. If these conditions are 

 fulfilled seedlings will grow healthily and the method of 

 their growth can then be studied. 



Direction of Growth.-^-The first observations to be 

 made are on the direction of growth of the various 

 parts of the seedling. It is noticed that the root makes 

 its way down into the soil, while the shoot grows up 

 into the air. Further investigations will determine the 

 causes which bring about this directive growth. 



Experiment 56 



Aim. — To show the direction of growth taken by the 

 root and by the shoot of seedlings when the seeds are 

 planted in various positions. 



