GROWTH 115 



After a time young bulblets will form at the base of each 

 scale. Weigh the scales again, and if there has been any 

 loss, account for it (see Sections 24-27, and 65). The 

 same experiment can be tried by allowing hyacinth or 

 other bulbs to germinate without absorbing moisture 

 enough to affect their weight. 



156. Conditions of Growth. — The internal conditions 

 depend upon the organization of the plant. The essential 

 external conditions are : food material, water, oxygen, and 

 a sufficient degree of warmth. It may be greatly influ- 

 enced by other circumstances, such as light, gravitation, 

 pressure, and (probably) electricity, but the four first 

 named are the essential conditions without which no 

 growth is possible. 



157. Region of Growth. — It was seen in Sections 148 

 and 153 that the region of active growth in the root is 

 just above the tip, behind the cap. In the stem the 

 region of increase is more evenly distributed, the lower 

 nodes continuing to grow for some time after the others 

 are formed, but a little observation will show that in stems 

 also, growth is usually most active in the region near the 

 apex, where ne>v cells are being produced. 



158. Cycle of Growth. — When an organ becomes rigid 

 and its form fixed, there is no further growth, but only 

 nutrition and repair, processes which must not be con- 

 founded 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 thou- 

 sands of years. 



159. Direction of Growth. — Plant in a pot suspended as 

 shown in Figure 252, a healthy seedling of some kind, two 

 or three inches high, so that the plumule shall point down- 

 ward through the drain hole and the root upward into the 

 soil. Watch the action of the stem for six or eight days. 



