24 PHYSICAL EXPRESSION. 



expression of the vital effects of fertilization, or 

 sexual impregnation. Other cases of vital action 

 in plants are differently expressive. The action of 

 chlorophyll in the cells of a leaf is known by certain 

 chemical results carbonic acid gas being absorbed, 

 the oxygen being discharged from the plant, and the 

 carbon retained in its tissues. Here the chemical 

 changes are the expression of the action of chloro- 

 phyll. The growth and vital action of the yeast- 

 plant, when living in a solution of sugar, is in part 

 expressed by multiplication of the unicellular yeast- 

 plant a histological mode of expression and it is in 

 part shown by the chemical changes of fermentation 

 indicated by chemical results. 



Numerous kinds of movements occur in plants; two 

 examples will serve our present purpose, showing that 

 movements may be the expression of vital changes. 

 In plants, movement is effected by two principal 

 methods by unequal growth of the cells composing 

 the growing member ; or by organs termed "pulvini," 

 temporarily or permanently devoted to the produc- 

 ,tion of movement as their principal function. In the 

 Oxalis (wood-sorrel tribe), and in the sensitive plant 

 (Mimosa), movements of leaves are seen. In each 

 case the movements are effected by an arrangement 

 of cells at the junction of the leaf with its main 

 stalk; this group of cells is called a "pulviiius." The 

 pulvinus is the mechanism by which movement of 

 the leaf is effected ; it consists of a mass of small 

 cells destitute of chlorophyll, and therefore incapable 

 of performing any nutritive function in the plant, 

 or of taking any direct part in the elaboration of 



