Observations on Squamosis and Exanthema of the Citrus. 137 
which the grower must solve for himself. I may observe, however, that 
excessive humidity being harmful as regards gummosis only during the vege- 
tative period, it is quite evident that the distribution of the rainfall throughout 
the year will have to be considered in planning a drainage system. 
As regards methods of irrigation, the grower should aim at maintaining 
in his soil the proper moisture for growth by frequent irrigations. Infre- 
quent and copious irrigations should be avoided, as they produce recurrent 
periods of saturation favourable to the development of gummosis, which, 
under such a cultural regimen, if once induced is particularly difficult to 
cure. In heavy retentive soils flooding ought never to be used and should 
be replaced, where employed, by a furrow system of irrigation. In light 
well-drained soils flooding may, of course, be used without danger, which 
is fortunate, as frequently it is the only satisfactory method of irrigation for 
such soils. 
2. Resistant stocks. It was early observed that, ceteris paribus , the 
various species of Citrus differed widely in their resistant power to 
gummosis, but in Primus we have absolutely no reliable knowledge re- 
garding the susceptibility of the various species. It has, however, been 
pointed out by MTntosh that grafting the cherry on a stock that did not 
develop equally with the scion was more conducive to gummosis than when 
they were of equal growth, and there is no reason for assuming that the same 
observations would not apply to the other species of Prunus and Citrus. 
The relative susceptibility of the different species of Citrus to 
gummosis has only been empirically determined, and, consequently, the 
exact position certain of them should occupy is a matter of some doubt. 
One may expect, therefore, that careful study will reveal certain minor 
inaccuracies in the following resistance scale : — 
Citrus trifoliata , maximum resistance. 
C. amara 
Rough Lemon ( C.Limonum x C. decumana f) 
C. Limetta 
C. bergamia 
C. nobilis 
C. decumana 
C. Aurantium 
C. medica 
C. Limonum, resistance zero. 
intermediate resistance. 
As resistance stocks C. amara and Rough Lemon are the most used 
at the present day. It would seem that a priori the stock most resistant 
to gummosis should be used exclusively, but it is found in practice that 
another factor is of immense importance. The cultivated Citrus frequently 
form better trees on one stock than on another, hence the necessity of 
