[Chap. XXIII PLANT BEHAVIOR RELATED TO OSMOSIS 213 



be seen in a mass of soap bubbles. The walls of many cells are not 

 uniformly thick and equally extensible in all parts, and some of the curi- 

 ous shapes of cells may be attributed to this fact. 



It is important to remember that increase in size of a plant organ is 

 the result of the enlargement of its component cells. The mere division 

 of cells results only in increasing the number of cells that may enlarge. 

 A cell may become larger onlv bv the entrance of material from an ex- 

 ternal source. The great bulk of this material is water. 



Plant movements and curvatures. Some of the smallest plants are 

 motile and can swim about in water just as freely as small animals. But 

 the movements of larger plants are limited to the bending, twisting, or 

 elongating of certain organs or restricted parts of organs. 



Tropisms. Tropic movements, or tropisms, are exemplified by the fa- 

 miliar curving of plant organs toward, or away from, different inten- 

 sities of external factors, or the direction of the force of gravity or of an 

 electric current. For example, one-sided illumination subjects the cells 

 on opposite sides of a plant organ to different intensities of light. Simi- 

 larly the force of gravity or an electric current may induce curvatures. 

 If a plant organ curves toward the most intense light to which it is ex- 

 posed, its curvature is said to be positively phototropic. If it bends away 

 from intense light it is negatively phototropic. Similarly compounded 

 names are applied to curvatures related to other external factors, namely, 

 positively or negatively geotropic, hydrotropic, chemotropic, electro- 

 tropic, etc.; but the names are the least important features of the 

 processes. 



Tropic curvatures may occur in any plant organ. They occur pri- 

 marily and are most prominent in growing regions, such as the tips of 

 stems and roots and the petioles of young leaves. Moreover, they are 

 primarily restricted to that part of the growing region in which the 

 cells are enlarging. They are the result of unequal enlargement of the 

 cells on opposite sides of the plant organ. Instead of personifying them, 

 therefore, we may base our interpretations upon our knowledge of the 

 dependent relations of the energy of molecular motion and diffusion, 

 permeability of protoplasm, osmosis, cell turgor, and the extension of 

 cell walls. If the initial influence of some external factor results in a 

 decrease of turgor and cell wall extension, the plant organ will bend 

 toward that side to which the factor is applied, and vice versa. 



Hormones and cell enlargement. During the last decade the influence 

 of hormones upon cell enlargement and tropisms has become an inter- 



