360 LECTURE XV. 



growth of the cells lying at the same level is commonly not 

 uniform. These variations in the rate of growth are, like 

 those which we have already considered, spontaneous : they 

 take place when the external conditions are maintained as 

 constant as possible, for example, when temperature does not 

 vary and the plant is kept in darkness. 



The effect of these variations upon the direction of growth 

 of an organ will be made clear by the following considera- 

 tions. As the result of its growth, every organ takes up a 

 certain definite position. A straight line running through the 

 axis of the organ from its base to its apex will indicate the 

 direction in which its growth in length has taken place. If, 

 during the whole period of the growth of an organ the 

 growth of each transverse zone is uniform throughout its 

 whole extent, a line o\rawn through the axis of the growing 

 portion will at all times coincide with the prolongation of the 

 line drawn through the axis of the portion which has ceased 

 to grow. But if; in one or more zones, the growth of one 

 portion be greater for any time than that of the remainder, 

 the line drawn through the axis of the growing portion will 

 not at that time coincide with the prolongation of the line 

 drawn through the axis of the portion which has ceased to 

 grow, but will at some point form an angle with it. This 

 deviation of the two lines constitutes the Nutation of the 

 organ, and the extent of the nutation is measured by the 

 size of the angle at the point of intersection of the two lines. 

 Let us> to illustrate these statements, take the case of an 

 erect growing stem. A line drawn through the axis of that 

 portion of the stem which has ceased to grow, is vertical. If 

 all the transverse zones of the growing portion are growing 

 at a uniform rate throughout their whole extent, the line 

 drawn through the axis of the growing portion will also be 

 vertical. But if certain portions of one or more zones lying 

 together at one side are growing more rapidly than the re- 

 maining portions, the apex of the stem will be tilted out of 

 the vertical away from the side on which growth is most 

 active, and hence the lines drawn respectively through the 

 axes of the parts of the stem which have and have not ceased 



