CH. irr. DOWNWARD GROWTH OF THE ROOTS. 93 



the wires without touching them. One pressed con- 

 stantly against the wires, and seemed only compelled 

 to its course by the mechanical resistance from above. 

 I placed a string for each plant to climb, and as they 

 ascended I let the flower -pot down, so that at last it 

 touched the floor of the room, and the plants the 

 ceiling. They circle round their support against the 

 sun, as it is called ; that is, the course of their growth 

 and of their sap in the half circle on the south side of 

 their supports is from west to east, and in the half 

 circle on the north side of their supports from east to 

 west : in other words, they make the half circle which 

 is farthest from the observer from right to left, and the 

 half circle which is nearest to the observer from left to 

 right. 



Many plants do the same, and many others (as the 

 hop) exactly the reverse. JSTothing can alter these 

 determinations of growth in these plants, which are, 

 possibly in all cases, owing to the action of the same 

 external agent, light, on different internal cellular 

 organisation. 



Mechanically^ I have forced the heads of plants to 

 grow downward, by placing the seeds and roots of 

 beans in sponges, and confining their heads in glass 

 tubes. I have mechanically forced the first or tap- 

 roots of plants to grow upward by placing horse- 

 chestnuts in earth, or half covered with water, and 

 confining their tap-roots in glass tubes. Both the 

 stems and the roots will, however, make every attempt 



