PLANT PHTSIOLOar. 99 



when, as in the experiments above (Figs. 53, 53), there are 

 two lines of force acting at right angles to each other, the 

 axis of polarity is diagonal, and the cells assume a diagonal 

 position. 



173. The action of the plant in response to such forces 

 is known as Geotropism (see par. 186) and careful study 

 has shown that it is by no means confined to vertical stems 

 and roots. Many stems grow as persistently in a hori- 

 zontal as ordinary ones do in a vertical direction. So, also, 

 many roots grow almost at right angles to the controlling 

 force (gravitation, or centrifugal force). 



Practical Studies. — («) Plant seeds half an inch deep in a flower- 

 pot (Fig. 51), cover with coarse netting, and invert upon a ring-stand. 

 Below it place a mirror, standing at a proper angle to reflect light 

 upon the under surface of the flower-pot. Place a tall bell-jar over 

 the apparatus and keep water in the dish, so as to preserve a moist 

 atmosphere. Now place the whole in a light room of the proper 

 temperature. Upon germination the roots will appear below, while 

 the stems will grow upward into the soil. 



(S) Slip two small flasks containing a little water over opposite 

 ends of a wooden rod and retain them in place by a coil of wire, as 

 shown in Fig. 52. A spjouted seed is previously fastened to each 

 end of the rod by a stout pin, and the whole is then rotated rapidly 

 upon the steel rods by a water or electric motor Note the direction 

 of the roots and stems. 



(c) Construct the rotating apparatus shown in Fig. 53. Upon a 

 knitting-needle fasten a cork, in which are placed diagonally eight or 

 ten strips of mica (m); near its upper end fasten a second cork, and 

 cover with a bell-jar (ft); support the needle upon the centre of a 10- 

 cm. tube (4 in.) which is 60 to 100 cm. long (2 to 3 ft.). Fasten seeds 

 to the upper cork by pins, and place a Bunsen burner under the tube 

 to rotate the wheel. 



(<f) Construct a rotating wheel (Fig. 54), using a knitting-needle for 

 the axis, and a brass wire on which are strung corks for a rim. At- 

 tach the seeds to the corks by pins, and place it under a fine jet of 

 water. 



(e) Put plants in various unusual attitudes in a dark room, and 

 observe the positions assumed by the leaves and stems. 



(/) Germinate beans, and after the radicles have protruded a cen- 



