CHAPTER IX 



THE PECULIAR POWER POSSESSED BY PLANTS TO ADJUST 

 THEIR INDIVIDUAL PARTS TO THE IMMEDIATE SUR- 

 ROUNDINGS 



Irritability 



F the reader at this point will turn back to the Table 

 displaying the plan of this book, he will see that we 

 have now reached the end of our survey of the processes 

 concerned with the nutrition of plants. These proc- 

 esses are primarily internal, but they are all more or less depend- 

 ent, especially for their supply of material or power, upon some 

 one or the other of the external conditions. Now these external 

 conditions, — heat, light, water, minerals, and so forth, — are never 

 distributed quite uniformly around any individual plant, but are 

 more or less abundant in some spots or directions than others. 

 Obviously it would be a very great advantage to plants if each 

 separate one of their parts, — each leaf, stem, root, and so forth, — 

 could be adjusted or swung individually into the direction or 

 position that would enable it to work to the very best advantage 

 under the conditions presented by its own immediate surround- 

 ings. Such a power, and in high degree of efficiency, plants in 

 fact do possess, as we shall now proceed to consider. The reader 

 will be surprised, I predict, by the importance and interest of the 

 phenomena which belong under this head. 



We may best begin our study of the subject by considera- 

 tion of its most familiar example. When a potted plant, like a 

 "Geranium," is grown in a greenhouse lighted evenly all around, 



it assumes a symmetrical form, alike on all sides, as everybody 



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