GROWTH 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 i mm. spaces of a root of Vicia 
is here shown: 
I II III IV V VI VII VIII IX X 
1.5 5.8 8.2 3.5 1.6 1.3 0.5 0.3 0.2 o.i 
1 Transpiration may be another factor; the precise relation of the two is uncertain. See 
Part III, p. 536. 
