242 Plant Biology 



Polarity (L. polus, pole) is a phenomenon in which there exists struc- 

 tural and functional direction due to complex internal factors. For 

 example, experiments show that certain plant stems (such as willow, 

 etc.) when cut into sections and suspended in humid air will develop 

 shoots from the distal end and adventitious roots from the proximal end. 

 This proves that these stems possess a permanent physiologic difference 

 between the two ends which is called growth polarity. If the experi- 

 ment is performed in moist soil, roots may form on the original distal 

 end of the stem (when placed in the soil) but they will form more 

 slowly and less extensively than on corresponding stems whose proximal 

 ends are placed in the soil. So these stems seem to have a prospective 

 "shoot end" and prospective "root end" which shows polarity in the stem. 



Polarity seems to be present in individual cells, parts of organs, entire 

 organs, etc., in which functional polarity accompanies structural polarity. 

 Hormonal polarity exists in which the movements of plant hormones 

 (auxins, etc.) are primarily polar, taking place primarily from the more 

 distal (apical) to the more basal (proximal) parts of a plant structure. 

 This distribution and presence of hormones in certain regions of plants 

 explain some of the many growth and behavior phenomena of plants. 



Electrical polarity is experimentally proved in which the distal (apical) 

 end of stems is electropositive, while the basal (proximal) part is electro- 

 negative. A similar electrical polarity exists in cells. All of these polari- 

 ties seem to be inherent and usually fixed and ordinarily cannot be 

 changed materially by environmental conditions. 



When living cells pass through their enlargement stages, they undergo 

 differentiation (L. differe, to differ) in which division of labor and dif- 

 ferences in structure and form occur, depending on the various functions 

 to be performed. These causes of differentiations are due to hereditary 

 determiners in each species of plant, being transmitted from one genera- 

 tion to the next. Environmental conditions, at times, may modify these 

 differentiations of cells but only quantitatively and not permanently. 

 This study of differentiation is called morphogenesis (mor fo -jen' e sis) 

 (Gr. morphe, form; genesis, origin). 



XII. PLANT TROPISMS (REACTIONS) 



Each species of plant is affected in specific ways by external and in- 

 ternal factors. Light, heat, moisture, chemicals, gravity, and atmos- 

 phere are a few of the influential external factors, while the chromo- 

 somes and their genes, the chemical constituents of the protoplasm, and 

 the chemical hormones are important internal factors. External en- 



