THE FOOD OF PLANTS. 135 



rophyll by any other metal, but Naegeli's researches have 

 shewn that it can be to some extent replaced by Strontium, 

 Barium, or Magnesium, in the food of certain Fungi (Moulds). 

 It is especially abundant in the leaves of green plants. The 

 effect of an insufficient supply of calcium is shewn in Fig. 

 21,4. 



It was pointed out in the second lecture (p. 22) that cal- 

 cium occurs commonly in the cell-wall, and it is well known 

 that it forms compounds with proteids. It is therefore possi- 

 ble that it contributes to the building up of the tissues in the 

 form of organic compounds. 



Calcium very commonly occurs in the cells of plants in the form of 

 crystals of the carbonate or oxalate, and it is possibly one of the im- 

 portant functions of calcium to form insoluble salts with acids which are 

 of no further use in and are even injurious to the plant. 



Magnesium, like calcium, may be advantageously absorbed 

 by plants in the form of all its salts except the chloride. Very 

 little is known as to its use. Its distribution appears to be 

 tolerably uniform. It appears, from Naegeli's observations, 

 that it can be replaced in the food of Moulds by calcium. 



Magnesium occurs in the aleurone-grains of seeds in combination with 

 phosphoric acid and calcium in the form of rounded masses termed 

 globoids (see infra]. 



Iron. It appears that iron may be absorbed by plants in 

 the form of the most different compounds. 



With regard to its use in the plant it may be mentioned 

 in the first place that iron has been detected in the most 

 different plants and in the various parts of plants, either in 

 the cell-contents or in the cell-wall, but it has been found to 

 be essential only to those plants which contain chlorophyll. 

 If a seedling be cultivated by means of the method of water- 

 culture, with its roots in a solution which contains no iron, 

 the leaves which are formed will be successively paler in 

 colour until at length they are nearly white ; in this state the 

 plant is said to be chlorotic. If now a small quantity of a salt 

 of iron be added to the solution in which the roots are im- 

 mersed, or if the white leaves are painted with a dilute solu- 

 tion of iron, they will very shortly become green. Iron plays, 



