26 PHYSICAL EXPRESSION. 



in growth, and destitute of chlorophyll. This is the 

 condition of the cells of the pulvinus as long as it 

 is capable of producing movement. In the Oxalis 

 corniculata the pulvinus is developed imperfectly, 

 and to an extremely variable degree, so that it is 

 apparently tending towards abortion. Its cells con- 

 tain chlorophyll ; that proves they have nutritive 

 functions to perform, as well as the production of 

 movement. These cells are more like the normal 

 cells of the petiole than those usually characteristic 

 of a pulvinus.* 



As to the production of movement in plants by 

 unequal growth of the cells. In the process of 

 germination of the pea seed, the radicle of the 

 embryo protrudes from its case, and direct ex- 

 periment demonstrates that the apex of the root in 

 its downward progress does not proceed in a straight 

 line ; the apex bends first to one side, then to another, 

 moving in irregular ellipses, or rather in a spiral 

 direction, as it descends. This zigzag movement of 

 the apex of the root is due to an unequal growth 

 of the cells composing the growing portion of the 

 root; they enlarge first on one side, thus causing the 

 apex to bend towards the opposite direction, and 

 then the set o cells growing alters. This mode of 

 growth is termed by botanists "circumnutation." The 

 essential principle of this mode of growth is, that 

 the vegetable cells concerned do not grow together, 

 but those on one side grow, then those at another 



* " Analogy between the Movements of Plants and the Muscular 

 Movements of Children, called Chorea," F. W,, British Medical 

 Journal, February 25, 1882. 



