48 BOTANY 



while the stems are negatively geotropic, is of the greatest 

 importance to an ordinary plant, for the function of 

 the roots is to grow into the soil to hold the plant and 

 to absorb water from the moist earth, while the function 

 of the stems is to grow out into the air and carry the 

 food-producing leaves into the light and air, where 

 they get the essentials for their manufactures. 



Another physical factor to which plants are sensitive 

 is the temperature. Heat and cold have a great in- 

 fluence on the growth and activity of all the parts. 

 Roots which are chilled cannot absorb water, and it 

 will be remembered how essential that is for the well- 

 being of the individual. On the whole, most vegetation 

 responds favourably to a comfortable warmth. But 

 the range of temperature is not very great, and ex- 

 cessive heat is bad for, and finally kills, most plants, 

 except those strange little algse which inhabit hot 

 springs. 



The fact that both cold and heat are bad and a nice 

 medium warmth is the most favourable temperature 

 for the general life, illustrates one of the interesting 

 results of a scientific study of plant physiology. A 

 similar, though not nearly so easily noticeable, series 

 of processes is observed in relation to light. Dark- 

 ness stops the food-forming activities of leaves (as well 

 as affecting the tissues in other ways) and light en- 

 courages it. But this light must not be too strong or 

 it is again harmful. True, in England, our plants do 

 not generally get any opportunity of experiencing this, 

 for the light intensity on these islands is not high ; 

 still experiments can be made with artificial light, and 

 it is found that when the light becomes very intense 

 it destroys instead of assisting the life functions. It 

 ia found, therefore, that there is a minimum quantity 



