112 



WATER-ABSORPTION AND TRANSPIRATION. 



These crystals (Fig. 40) are more abundant in leaves exposed to full 

 sunlight than in shaded leaves, and in the green than in the not-green 



parts of variegated leaves. 



The disappearance of nitrate from 

 leaves shows the same relation to 

 light and to the presence or absence 

 of chlorophyll as does the accumula- 

 tion of oxalate of lime. Nitrate dis- 

 appears from cut leaves in a few 

 days, if they are exposed to light; 

 f i>. <$$$*& ]} in variegated leaves the nitrate dis- 

 appears only from the green parts. 



B 



Fig. 40. Cells containing Crystals of 

 Oxalate of Lime of Different Shapes. 



148. Formation of Proteids. 



Thus we have explained, as far as 

 possible, the building up of soluble 

 carbohydrates (sugars) and soluble 

 nitrogenous compounds (amides) in 

 the green leaf. With the exception 

 of what is used directly by the assimi- 

 lating cells, the sugars and amides are transferred to the various 

 parts of the plant. All living cells contain these soluble carbohy- 

 drates and nitrogenous compounds conveyed to them in the cell-sap. 

 The living protoplasm makes use of these as food substances. Together 

 with sulphur and phosphorus they are built up first of all into more 

 complex (proteid) substances, and, finally, into protoplasm. 



One of the chief forms in which sulphur enters the plant is calcium 

 sulphate. The sulphur is liberated, and thus enabled to combine with 

 the organic food substance, by the action of an organic acid. This acid 

 appears in many cases to be oxalic acid. The calcium of the sulphate 

 combines with the oxalic acid to form calcium oxalate. 



149. Use of the Metallic Elements. Potassium, calcium, mag- 

 nesium, and iron do not enter into the composition of the living 

 substance, yet they are essential elements. Now we have seen that 

 iron, although it does not enter into the composition of chlorophyll, is 

 necessary for its formation. This gives us a hint as to the use of the 

 other elements. It would seem that potassium in the same way is a 

 necessary condition for the formation of carbohydrates, and that 

 calcium and magnesium are necessary for the distribution of carbo- 

 hydrates. Calcium is also of importance in the formation of more 

 complex substances from carbohydrates, since it combines with, and 

 thus makes harmless, the poisonous by-product (oxalic acid) which is 

 formed in these processes. 



150. What are the Chief Functions of the Boot? 



Refer to the parts of Chapter I. in which the root of the 

 Broad Bean is dealt with, go over the observations you have 

 made on the roots of seedlings which you have grown, and 



