NATURE OF PLANTS 89 



tions aroused in the plant by various stimuli. The entire plant 

 responds as a unit to these forces. Stimulation and the conse- 

 quent growth in one part has its effects upon all other parts. As 

 a result of this interaction there comes about a correlation in the 

 growth of all the parts that is expressed in the symmetry of the 

 plant body and in the perfect adjustment of roots, stem and 

 branches, and leaves to its needs. Any window garden, forest, 

 or field furnish abundant evidence of the complicated and varied 

 nature of these reactions. An illustration of this is afforded in 

 the removal of the terminal axis of a tree. An adjoining lateral 

 branch that heretofore has been stimulated to a more or less 

 horizontal growth, now is so directed in its development that 

 it ultimately becomes the terminal axis of the tree. So some 

 parts are constantly being overshadowed and choked out while 

 others are directed into favorable positions and perform the 

 work of the destroyed parts. In this way there is brought about 

 that weaving and twisting of stems and branches and the ad- 

 justment of leaves that so often suggest a conscious effort to 

 reach favorable positions. 



43. Modifications of the Stem. — In the above discussion at- 

 tention has been called to the more common characteristics of 

 the stem. It is well to remember, however, that variation is 

 the law in the plant world. No two plants are alike. The 

 environment of the plant is constantly stimulating it and caus- 

 ing it to vary. Frequently the variations are so minute as to 

 escape our attention or totally distinct forms may arise. These 

 departures from the parent type may be of no advantage to the 

 plant and it may therefore perish. On the other hand the varia- 

 tions may be of such a character as to enable the plant to more 

 successfully compete with other plants and as a consequence the 

 plant with its helpful variations will survive. So it has come 

 about that these changes going on during the past ages have 

 resulted in many remarkable modifications. A few of the modi- 

 fications of the stem that are of especial advantage to the plant 

 and therefore spoken of as adaptive variations will be considered 

 in the following paragraphs. 



44. The Climbing Type of Stems. — Climbing stems are char- 



