1 84 THE TEACHING BOTANIST 



must be imported from Germany by one of the dealers in labor- 

 atory supplies (see page 93). It is useful for many purposes, 

 however, and is a profitable investment. I have made a fair 

 substitute, as follows : buy a Seth Thomas eight-day clock, cost 

 about $5 ; have a watchmaker alter the wheels so that the 

 spindle of the minute-hand will make a revolution in about 

 fifteen minutes (rendered necessary by the shortness of " re- 

 action time ") ; this can be done by shortening the hair-spring 

 and removing each alternate tooth from the escapement wheel ; 

 let him make a brass disk two inches in diameter, with holes 

 on its edge, and an arm to slip over the spindle, so that the 

 disk will revolve parallel to the face of the clock. To this 

 disk may then be fastened, by tacks through the holes, a cork 

 bearing the seeds, as recommended above (Fig. 17). Such a 

 clinostat will not carry large flower-pots, but seeds and seed- 

 lings grown in moss show the principle of irritability as well as 

 potted plants. 



From observation of this experiment, in which the two sets of 

 seeds are under precisely the same external conditions in every 

 respect except in their relations to gravitation, students should 

 be able to deduce the fact that gravitation is a determining in- 

 fluence in giving the up-and-down position to the developing 

 plumules and roots. Since, however, parts grow up against 

 gravitation as well as down with it, it cannot act simply as 

 " weight," but can only serve as a guide or index of direction. 

 The teacher will have to explain that the movement of the 

 growing parts around into the vertical position is brought about 

 by one-sided growth, a subject very easily illustrated by experi- 

 ment. Gradually the students may be brought to recognize in 

 the process the three elements: (i) an hereditary knowledge 

 (as it were) in the embryo of the advantageousness of sending 

 stems up and roots down; (2) a power of perceiving from the 



