94 



BOTANY OF THE LIVING PLANT 



fruiting. But the experiment may be varied by the omission of any 

 one of those elements. Any constant change in the condition of the 



plant following that omission may be 

 held as evidence of the part that 

 substance plays in its normal physio- 

 logy. This is most readily exempHfied 

 in the case of Iron. If the solution 

 be made up with chemically pure 

 materials, the result of growing it 

 without Iron is that after the first 

 leaves, which may appear pale green, 

 the later leaves develop yellowish. If 

 carried far the subject of the experi- 

 ment will appear starved. If after 

 this pale yellowish, or chlorotic state 

 is well established a trace of Iron Salt 

 be added to the solution, the normal 

 green colour will be developed, appear- 

 ing first along the veins ; or even if it 

 be applied locally to the surface of the 

 leaves, the full green colour will appear 

 at those points where the Iron Salt has 

 been applied. The conclusion is that 

 Iron is essential for the formation of 

 the green colour, and that a very 

 small quantity of it suffices. Similar 

 experiments may be made by omit- 

 ting other elements from the recipe, 

 for instance Potassium (Fig. 70): Such 

 results serve to show how important 

 the Transpiration-Stream is in supply- 

 ing the necessary salts from the soil. 



Fig. 70. 



Water-culture of Buckwheat. I. shows 

 a culture where a full nutrient solution 

 was supplied. II. shows the result of 

 the omission of potassium. (From Stras- 

 burger, after Nobbe.) 



Water-Economy. 



The maintenance of the Transpiration-Stream is also important for 

 the preservation of mechanical rigidity in the succulent parts. The 

 flaccid and limp condition of the withering shoot, and the recovery 

 which it shows on sufficient water-supply, are matters of common 

 knowledge. Both are referable to the turgor of the individual cells, 

 which falls when water is deficient, and the firmness of the whole tissue 



