AN INTRODUCTION TO PLANT BIOLOGY 119 



Although these responses of plants to various stirmili 

 have been observed and studied for a long time, we d 

 understand them. Neither do we know why a frog respo 1 

 when touched. We simply know that it has the power of 

 such response and that in some way it is connected with the 

 protoplasm in certain special nervous organs of the frog. 

 Likewise, we know many facts about plants responding in 

 ways very similar to animals and that this power of response 

 is connected with the protoplasm of plants, but apparently 

 in no particular organs like nervous organs. We use the word 

 irritability to mean that animals and plants respond to 

 stimuli. We do not know what it is, but there is plenty of 

 evidence that such a power exists, and we know much about 

 how it works. Likewise, we know that electricity exists, 

 very much about how it acts, and can make great use of 

 it ; and yet no one knows what electricity is. These are 

 merely examples out of hundreds of cases in science where 

 we must accept the facts and make use of the knowledge 

 which it is possible to discover even when we cannot find 

 a satisfactory explanation. 



109. Summary of Work of Plant Organs : Life-Processes. 

 Most plants require, as food-material, carbohydrates, 

 nitrogen compounds, and certain other elements. Plants with 

 chlorophyll can make the necessary carbohydrates by com- 

 bining the elements from carbon dioxide and water. Light 

 and chlorophyll are essential for making carbohydrates. 

 Plants which have no chlorophyll must absorb the carbo- 

 hydrates (probably as sugar) from decaying organic matter. 

 The nitrogen, in the form of compounds with other necessary 

 elements, are commonly absorbed from the soil or water in 

 which the roots grow. ' 



All plants require oxygen for use in the oxidation which is 

 going on constantly in all living cells. Plants with chlorophyll 

 may get oxygen from that which is freed from carbon dioxide 

 when the carbon is used in starch-making. For this reason 



