METABOLISM DIGESTION TRANSLOCATION GROWTH 2 17 



in internal growth and the making of foods. Many tropical 

 plants also have more or less well-defined seasons of rest. How- 

 ever, some plants would grow indefinitely if the weather condi- 

 tions were favorable ; we have already called attention to some 

 of these plants with indefinite growth (see page 42). 



(4) It is well known that many plants grow more rapidly 

 during the night than during the day. This may: be due to 

 the fact that intense light has a tendency to inhibit or check 

 plant growth. However, the carbohydrates which the plants 

 use to provide energy for growth were made in the presence 

 of sunlight. 



(5) The most important environmental factors which in- 

 fluence or control plant growth are moisture, temperature, sun- 

 light and character of soil which have already received considera- 

 tion in the preceding chapters. 



Irritability. A comparison of plants growing under differ- 

 ent environments, under favorable and unfavorable conditions, 

 is evidence of the response of the internal activities of the plant 

 to the external or environmental factors. Since the protoplasm 

 is the center of all activity on the part of the plant or any other 

 living organism, we naturally turn to it for an explanation of 

 these phenomena. It can be readily demonstrated by experi- 

 mental work that the protoplasm of the active cell responds to 

 stimulation with heat, light, chemicals, electricity, etc. This 

 can also be observed on the growing organs of the higher plants, 

 which respond to various environmental factors; roots seek 

 darkness and moisture, while the leaves turn towards the sun- 

 light, the entire plant responding to these and many other fac- 

 tors, such as mineral foods and temperature. 



The twining of such plants as the morning-glory and the 



