54 METHODS OF INVESTIGATING 



areas is then most easily recognizable, while if one simple leaf 

 or leaflet contains both permeated and control areas a difference 

 in the rate of growth of the two parts of the leaf will result in 

 a puckering of the leaf which is readily observed. 1 



These conditions are fulfilled when leaves of certain species, 

 as, for example, apple, pear, plum, strawberry and broad bean, 

 are subjected to inter venal injection. For this treatment a small 

 incision is made near the midrib of the leaf between two major 

 secondary veins. A dilute solution of the salt of the element of 

 which a deficiency is suspected is contained in* a small tube, 

 and a wick made of filter paper, or, for small leaves, of 

 darning cotton, passes from the solution through the incision 

 into the leaf. By the use of dyes it is shown that the solute 

 diffuses through the whole area between the two secondary veins 

 and the leaf margin before diffusing into neighbouring inter- 

 venal areas. The length of time for which injection is allowed to 

 proceed should be such as to give a long boundary between the 

 injected and neighbouring control area, but not so long that the 

 solute diffuses into neighbouring areas. The best time must be 

 found by preliminary trials, for it varies with the species and 

 climatic conditions. However, for apple, pear, strawberry and 

 Shasta daisy, Roach suggests a period of from 7 to 12 hr. The 

 leaves which give the best response are those about half -grown. 

 The maximum response is generally given in about 10 days, but 

 a response has been observed in as short a time as 2 days. This 

 was recorded by Roach as having been observed by Lai as a 

 result of intervenal injection of soya-bean leaves with a 0*025 

 per cent solution of ferrous sulphate. 



In leaf -tip injection the tip of a leaf or leaflet is cut off at right 

 angles to the midrib and the cut edge of the leaf immersed in a 

 solution *of the substance to be injected. This method can be 

 used for any type of leaf but is particularly suitable for long 

 narrow leaves. The greater the proportion of the leaf removed, 

 the greater is the penetration of the solute into the rest of the 

 leaf. For example, it was found that if the removed tip con- 



1 It should, perhaps, be pointed out that these responses should only 

 be regarded as indicative of deficiencies when they have been correlated 

 with successful curative treatment, since an improved appearance of 

 the leaf might result from injection without there being a deficiency. 



