CHAPTER IX. 



GROWTH, IRRITABILITY AND MOVEMENT. 



In the preceding chapter we considered the processes of synthesis in 

 which raw materials derived from the environment are built up within 

 the plant into substances such as sugars and various organic nitro- 

 genous compounds. Except for the proportion that is consumed in 

 respiration, these products of synthesis are ultimately utilised in the 

 formation of new tissues, leading to the growth of the plant. We 

 have now to consider various aspects of growth processes in the plant 

 and of associated phenomena. This includes a study of the sensitive- 

 ness or irritability which growing organs of the plant exhibit towards 

 certain directive influences to which they are exposed during their 

 development. And it will lead to a general consideration of phenomena 

 of movement in plants. 



I. Growth. 



Growth is one of the most conspicuous features of Life. Its most 

 obvious sign is increase in Size. But as applied to a living organism 

 the term means something more than mere enlargement. It involves 

 change and transfer of materials which cannot by any means be 

 restored to their original state. Nor is it merely that the existing 

 plants or parts of them enlarge as seen from without : for additional 

 organs are successively developed, so that growth of the plant 

 commonly leads to an increasing complexity of form, and also of 

 internal structure. 



I. Germination. 



In entering on the studv of Growth it will be convenient to con- 

 sider it as it is seen in Flowering Plants, and to trace it from the 



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