20, 3 McLean and Lee: The Citrus-canker Organism 318 
citrus leaves, using the apparatus described above.’ The re- 
sults of these determinations showed clearly that the permeability 
varies greatly for leaves of the same age and on the same tree, 
so that large numbers of readings are necessary to establish 
satisfactory averages. 
The Szinkom mandarin orange variety employed in the ex- 
periments is a variety successfully grown in the Philippines ; al- 
though introduced here from India, it is believed to be a native 
of China. It is one of the most canker-resistant horticultural 
varieties of any of the citrus species. The grapefruit trees em- 
ployed in the experiment were in a few cases of the Pernambuco 
variety, and in others of a seedling variety, all rather susceptible, 
according to field observations. Since grapefruit leaves were 
not sufficiently abundant, a susceptible pummelo variety was 
chosen to complete the comparative tests. The pummelo trees 
were of the East Indian type, also known as shaddocks; they 
also were matured seedlings. In the field, from the amounts 
of natural infection, one would judge these trees employed 
in the experiments to have much the same degree of suscep- 
tibility as the grapefruits. The results with four Citrus 
varieties are summarized in Table 1. 
TABLE 1—Results of application of pressure on leaves of Citrus species 
immersed in water. 
a ; 
Pressures causing injection. 
Variety, Tests. 
Average. | Highest. | Lowest. 
cm, cm, em. 
Pernambuco grapefrait ..2... 202202222 oe 60 19.5 53.0 10.0 
East Indian pummelo Bgl aries a ic ep ee 130 19.6 55.0 5.5 
Washington navel orange .......--.--.--.-------.-------- coe 20.8 50.5 9.0 
Szinkom mandarin orange._..__-__-----------c----ee--ee 18 33.6 55.0 11.8 
The range of values from the highest to the lowest seems 
to be about the same for all varieties tested. The highest value, 
55 centimeters, represents the highest mercury column attain- 
able with the apparatus, and a number of leaves failed to be 
injected even with this extreme pressure. The average values 
appear to be more significant, the more-susceptible grapefruit 
and pummelo being the easiest to inject and the highly resistant 
Szinkom mandarin orange the most difficult, the Washington 
* Most of these tests were made by S. Bacol, R. Cuitiong, M. Punzalan, 
F. Esguerra, F. Rodis, and T. Bautista, all advanced students in plant 
nutrition, working under the direction of the senior author. 
