54 



TEXTBOOK OF BOTANY 



By examining a vertical section of a growing stem tip ( Fig. 32, page 69 ) 

 one may see that growth in length of a plant stem is a consequence of 

 ( 1 ) the formation of new cells by cell division at the apex of the stem 

 and (2) the subsequent enlargement of these cells. As a result of these 

 two processes the stem elongates. 



Fig. 27 shows the familiar fact that potato sprouts grow in length more 

 rapidly in darkness than in bright light. How shall we account for this? 



Fig. 28. Differences in stems and leaves of bean seedlings that grew in different 

 light intensities: 100%, 50%, 0%. Photo by F. H. Norris. 



The sprouts that grew in the dark have comparatively small leaves and 

 lack the familiar green color. Fig. 28 illustrates bean seedlings that grew 

 in bright light, in shade, and in darkness. The stems of the seedlings 

 that grew in shade and in darkness are longer than those that grew in 

 bright light. The seedling from the dark box has small leaves and lacks 

 chlorophyll, but the seedling in the shade is green and has larger leaves 

 than the one in bright light. Hence, the green color and the size of the 

 leaves do not account for the differences in the growth in length of the 

 stems. 



Still more exact data have been obtained. An experimenter found that 

 certain tobacco plants grew in height 3 times more rapidly at night than 



