GROW III AND MOVEMENT 421 



degrees, according to circumstances. • Thus, cells which are part of an 

 elongated organ like a stem, are likely to grow much more in the longi- 

 tudinal diameter than the transverse. The real reason for these ine- 

 qualities of growth is obscure. To say that they are due to " inherent 

 causes" or are determined by "heredity" in no wise enlightens the 

 inquirer. In a few cases they are referable to definite agencies. Thus, 

 the cells near the upper surface of a leaf are influenced, mainly by light, 

 to grow longer in the axis at right angles to the surface than in the other 

 two. 1 The sum total of growth in the individual cells determines in large 

 measure the final form of the organ in which they lie. In most cases the 

 causes which determine the general course of growth can be analyzed at 

 present as little as those which determine the form of the single cell ; 

 but the effect of external agents is often detected, and in many cases 

 it is dominant (see section 3, p. 435). 



Grand period. — Enlargement proceeds at an unequal pace, even 

 though the external conditions which affect the rate are kept uniform. 

 In the earlier portion of the period it is slow, then it becomes more and 

 more rapid until it attains a maximum, when it quickly falls off and 

 gradually comes to an end. If the progress is graphically represented 

 by plotting the increment from day to day, a curve is obtained of which 

 fig. 668 is an example. This is the history, indeed, of the growth in 

 length of a short portion of a stem, which is made up of a multitude of 

 cells in the phase of enlargement. In a similar way the growth in volume 

 of a fruit, such as an apple or a pumpkin, might be described. The 

 total period of enlargement is named the grand period of growth, to dis- 

 tinguish it from periodic variations in the rate within the grand period, 

 some of which are due to periodically acting external agents, such as 

 light and heat (daily period, see p. 436), and others to causes unknown 

 and hence called " spontaneous " variations. 



The same features of the course of growth may be seen when the increment of 

 successive small portions of an axis is recorded. Thus if a root is marked into milli- 

 meter spaces, or a stem into longer spaces and the increment of each is recorded for 

 a number of hours, it will appear that certain spaces are growing more rapidly than 

 others, respectively more or less distant from the tip, i.e. older or younger. 



The increment in twenty-four hours of each of ten 1 mm. spaces of a root of Vicia 

 is here shown: 



1 Transpiration may be another factor; the precise relation of the two is uncertain. See 

 Part III, p. 536. 



