178 A TEXTBOOK OF BOTANY [On. IV, 9 



side branch of Spruce or Fir turns upward, and takes the 

 place of the main stem, on destruction of the main terminal 

 bud. All evidence shows that the geotropism of any part is 

 not in the least inherent in its morphological nature, but is 

 correlated with its habit of life. 



Stems and roots are by no means the only geotropic parts. 

 Thus, in the experiment of the inverted potted plant, per- 

 formed in the dark, not only does the stem grow upward, 

 but most of the leaf blades become horizontal, through in- 

 dependent readjustment upon their own petioles (Fig. 120). 

 Something similar occurs naturally in the undergrowth of 

 woods, where, in the evenly-diffused light, many kinds of 

 leaf blades become strikingly horizontal. Flowers and 

 fruits are often perfectly geotropic, as will later appear in 

 connection with their functions. The lower forms of plants 

 are likewise susceptible to geotropism; and the horizontal 

 growth of the bracket Fungi upon trees, and the upright po- 

 sition of the Mushrooms and Toadstools are thus guided. 



While geotropism thus guides the growth of plants in their 

 various parts, its effects of course are intermingled with 

 other irritable responses. Thus, when light acts from one 

 side on a geotropically upright plant, the position which the 

 plant actually takes is a resultant between its tendency to 

 respond both ways; and its roots and other parts are in- 

 fluenced in similar manner. The fact is, the plant tends 

 to guide its growth as a whole, and assume its general form, 

 under the guidance of gravitation, thus establishing a gen- 

 eral arrangement of parts, but this ground form is readily 

 modifiable in details by other influences. 



It is interesting to note the way in which the plant appears 

 to perceive, so to speak, the direction of action of gravita- 

 tion. There is much evidence to show that in special cells 

 of root and stem tips, the starch grains, which are relatively 

 hard and heavy, press down by their weight on the sensitive 

 protoplasm, and thus give the sense of direction by which 

 the growth processes are controlled. 



