50 PRACTICAL COURSE IN BOTANY 
nation: focd material, water, oxygen, and a sufficient. 
degree of warmth. It may be greatly influenced by other 
circumstances, such as light, gravitation, pressure, and 
(probably) electricity ; but the four first named are the essen- 
tial conditions without which no growth is possible. 
50. Cycle of growth.— When an organ becomes rigid 
and its form fixed, there is no further growth, but only nutri- 
tion and repair, — processes which must not be confounded 
with it. Every plant and part of a plant has its period of 
beginning, maximum, decline, and cessation of growth. The 
cycle may extend over a few hours, as in some of the fungi, or, 
in the case of large trees, over thousands of years. 
51. Geotropism.— The general tendency of the growing 
axes of plants to take an upward and downward course as 
shown in Exp. 37 — in other words, to point to and from the 
center of the earth —is called geotropism. It is positive when 
the growing organs point downward, as most primary roots 
do; negative when they point upward, as in most primary 
stems; and transverse, or lateral, when they extend horizon- 
tally, as is the case with most secondary roots and branches. 
52. Gravity and growth. — It cannot be proved directly 
that geotropism is due to gravity, because it is not possible 
to remove plants from its influence so as to see how they 
would behave in its absence. The effect of gravity may be 
neutralized, however, by arranging a number of sprouting 
seeds on the vertical disk of a clinostat, an instrument 
fitted with a cleckwork movement by means of which they 
may be kept revolving steadily for several days. By this 
constant change of position gravity is made to act on them 
in all directions alike, which is the same in some respects as 
if it did not act at all. If the disk is made to revolve 
rapidly, the growing root tips turn toward the axis of motion, 
without showing a tendency to grow downward. We may 
then conclude that geotropism is a reaction to gravity. 
53. Geotropism an active force.—It must be noted, 
however, that the force here alluded to is not the mere me- 
