50 PRACTICAL COURSK IN BOTANY 



nation : food 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 clockwork 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 th;it 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- 



