GROWTH AND MOVEMENT 421 



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

 elongated organ like a stem, arc likely to grow mm h 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 < ells near the upper surface of a leaf are influent ed, 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 < ases 

 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 lie seen when the in. remenl <>f 

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

 meter spa is, or a stem into longer spaces ami the in< remenl of ea h is ro orded for 

 a number <>f hours, it will appear that certain spaces are growing more rapidly than 

 others, respe< lively more or less distant from tin- tip, ;'./•. older or younger. 



The increment in twenty-four hours of each of ten 1 mm. spa esof a root 

 is here shown: 



1 Transpiration may Ik- another f.utor; the precise relation of tin- two is uncertain. See 

 fart III, p. 536. 



