44 GENERAL BOTANY 



and fruit are, therefore, determined by the three factors so 

 frequently referred to in the previous pages, namely, body 

 plan, bud growth, and adjusting movements, or tropisms. 



These few illustrations of the adjusting movements of some 

 common plants will enable the student to interpret similar 

 phenomena in the great variety of plant forms which enter 

 into his daily experience. The simple account given above 

 may also stimulate interest in the movements of plant parts, 

 and will furnish a clearer realization of the important factors 

 to which the ultimate forms and attitudes of common plants 

 are due. 



SUMMARY 



Plants, like animals, are sensitive to stimuli acting upon the living 

 substance of the plant organism. 



The nervous mechanism of plants is, however, lodged in the living 

 substance of growing organs, and there is no evidence of the differ- 

 entiation within the plant body of a definite nervous system corre- 

 sponding to the nerve fibers and nerve cells of the higher animals. 

 The reception of stimuli and the transmission of impulses in plants 

 take place, therefore, through the living substance of plant organs, 

 which is undoubtedly so connected as to establish a continuity of 

 living substance throughout sensitive and moving parts. 



The mechanism of movement also differs in plants and animals, 

 being confined in plants to growing parts in which the unequal 

 growth on different sides of an organ can effect curvature and move- 

 ments such as we have observed above in roots, stems, and leaves. 



The function, or use, of the sensitiveness of plants and their power 

 to react to stimuli is found in the adjustment of their organs to 

 the environment. For this reason plants have developed a special 

 sensitiveness to such stimuli as light and gravity, which enables 

 them to adjust their organs to advantageous positions for the 

 absorption of raw materials, the manufacture of food, and the produc- 

 tion of flowers, fruits, and seeds. Highly sensitive living substance 

 for the reception of stimuli is found in root tips, in some stem 

 tips, and possibly in some leaf surfaces. Specialized motor organs 

 are also developed in some plant families. Plants as a whole, how- 

 ever, while sensitive to a greater variety of stimuli than animals, 

 are far below the latter in special sense organs and in nervous organi- 

 zation for receiving and responding to external and internal stimuli. 



