ROOT-PRESSURE 



69 



108. The plant 

 thrives in a 

 weak solution. 



To test this matter, secure a young radish plant (or almost 

 any seedling with several leaves) and insert the roots into 

 a small bottle containing some of the saltpeter solution I. 

 In another bottle place a similar plant with 

 some of the weak solution II. Support the 

 plant in the mouth of the bottle with cotton 

 batting. After standing for a few hours or 

 less it will be noticed that the leaves of the 

 plant in the strong solution begin to wilt, as 

 in Fig. 107. The plant in the weak solu- 

 tion, Fig. 108, is rigid and normal. This 

 further indicates that the growing plant is 

 so constituted as to be able to make use of 

 very dilute solutions only. If we attempted 

 to feed it strong fertilizer solutions, these 

 strong solutions, instead of being absorbed 

 by the plant, take water from the latter, 

 causing the plant to wilt. 



145. The farmer or gardener knows that he can injure 

 or even kill his plants by adding too much plant-food. 

 Everyone recognizes the value of wood ashes as a ferti- 

 lizer ; but no one would dare water his valuable plants 

 with lye, or sow his choice vegetable seeds on an ash 

 bank, however well it might be watered. If there is a 

 potted plant at hand which is of no value, remove some 

 of the soil, add considerable wood ashes, w r ater well, 

 and await the result ; or give it a large lump of nitrate 

 of soda. 



146. ROOT-PRESSURE. The activity of the root in absorb- 

 ing water gives rise to considerable force. This force is 

 known as root-pressure. The cause of this pressure 

 is not well understood. ' The pressure varies in different 

 plants and in the same plant at different times. To 

 illustrate root -pressure, cut off a strong -growing small 

 plant near the ground. By means of a bit of rubber tube, 



