98 THE LIFE OF THE PLANT 



because it forms with another substance equally indis- 

 pensable for the nutrition of the plant, phosphoric acid, 

 a precipitate insoluble in water. We shake up this white 

 insoluble precipitate in the liquid, so as to bring it into 

 contact with the surface of the roots. We take several 

 jars : some with entirely clear solutions, which means 

 that they do not contain any iron salt ; others with a 

 certain degree of turbidity, owing to the presence of 

 the iron salt. Suppose we grow a plant such as maize 

 in each of these solutions. At the end of two or three 

 weeks we already notice a sharp difference between 

 them. While the full nutrient solution produces a 

 normal plant that will flower and produce ripe seeds, 

 the other will produce a plant with only a few narrow 

 and unhealthy leaves that will soon die altogether 

 (fig. 28). These leaves, moreover, will show a remark- 

 able peculiarity : the first two or three of them will be 

 of the usual green colour, but the rest will be white. 

 It is clear that the absence of iron has stopped the 

 development of the plant, and has resulted in a peculiar 

 disease, a ' pallid sickness,' called chlorosis. The 

 following simple experiment will attest the accuracy of 

 this inference ; we have only to add some of this iron 

 salt to the solution hitherto without it to see the 

 sickly condition coming to an end, the plant becoming 

 green and growing ; moreover, we have only to moisten 

 one part of a totally white and sick leaf with an iron 

 salt to see a green spot appearing soon after on that very 

 place. 1 Our attention has already been drawn more 

 than once to the similarity between the vital functions 

 of vegetable and animal organisms ; the action of the 

 iron salts presents a striking illustration of this point. 

 Such unfortunate cases as the following may have been 



1 In the middle of Fig. 28 we see a vigorous specimen of maize still in 

 flower reaching the top of the green house ; on either side are two 

 specimens of a smaller variety (Cinquantino) already bearing full-grown 

 cobs ; in between are two specimens grown without any iron salt. 



