WHITE CLOVER 125 



What do we mean by an " organ of movement " ? It 

 can generally be recognised at a glance by its form and its 

 position. At the base of a leaf -stalk, or some other 

 moveable part, we often find an enlargement or cushion. 

 A thin section at this point shows a crowd of minute, 

 colourless cells. The bundle of vessels, where it traverses 

 the cushion, becomes particularly flexible, owing to lack of 

 woody thickening in the walls of the vessels. The cover- 

 ing epidermis is often wrinkled, sometimes only on one side. 

 The small, crowded cells of the cushion are capable of 

 absorbing water from the surrounding tissues, on either 



FIG. 32. Three views of the organs of movement, at the bases of 

 the leaflets of a white clover-leaf. Magnified. 



side of the central strand. Then they swell, and the 

 whole stalks leans over to the opposite side. 



Proof that the organ of movement of the clover leaflets 

 is situated just where they are attached to the leaf -stalk 

 is not hard to find. We may destroy or injure other parts, 

 such as the blade of the leaflet, or the midrib, without 

 necessarily hindering the power of movement in what is 

 left. If the leaflets are amputated just above the base, 

 the stumps can still open and close. But if we destroy 

 or injure the part which has been described as the organ 

 of movement, the leaflets are unable to move any more. 

 Cut away with a pair of fine scissors the green blades from 

 all the leaflets of an expanded clover-leaf, and go with a 

 lantern after dark to see how they are behaving. You will 

 find that, notwithstanding the mutilation, the bare midribs 



