292 PHYSIOLOGICAL BOTANY. PART II. 



considered an anomalous circumstance. It is not 

 strictly true to say that the tendency of all stems is 

 upwards, though it is more nearly true that all roots 

 take a direction downwards. The branches of the 

 weeping birch, weeping willow, and some others of this 

 character incline downwards, merely by the effect of 

 gravity, acting upon the long slender rods of which they 

 are formed. But there are some trees, as the weeping 

 ash, and weeping horse-chestnut, whose branches take a 

 decidedly downward tendency from their very origin. 

 Many plants also have underground stems (fMoHMff), 

 besides those which they develop above ground. But, 

 neglecting these anomalies, it is generally true that the- 

 stem has a tendency to develop upwards, and the root 

 downwards. There are two causes to which we may 

 ascribe these modifications in the directions of the 

 stems and roots. One is " gravity," and the other 

 ' light." 



(300.) Effects of Gravity on Vi'tjctntinn. That 

 gravity is an important agent in determining the differ- 

 ence between the directions taken by the root and stem, 

 is shown by an ingenious experiment of Mr. Knight. 

 He placed some French-beans on the circumference of 

 two wheels, and so secured them that they could not be 

 thrown off when a rapid rotatory motion was given to 

 the wheels. One wheel was disposed horizontally, and 

 the other vertically, and both were kept in constant 

 motion whilst the beans were germinating. The radi- 

 cles of those beans which germinated on the vertical 

 wheel extended themselves outwards or from the cen- 

 tre, and the plumules inwards or towards it. Those 

 which were placed on the horizontal wheel pushed their 

 radicles downwards and their plumules upwards; but 

 the former were also inclined from, and the latter to- 

 wards the axis of the wheel. This inclination was 

 found to be greater in proportion as the velocity of the 

 wheel was increased. Now in the vertical wheel the 

 effects of gravity were nullified, since the beans were 

 constantly changing their position with respect to those 



