Root Development in Aluminous Soils — Plucknett, Moomaw, and Lamoureux 
405 
should result in stimulated root growth. An ex- 
amination of results of liming treatments shows, 
however, that although pH was increased and 
extractable A1 and plant A1 concentrations were 
decreased by liming, no marked stimulation of 
root development or plant growth of L. glauca 
occurred with liming. The increased root de- 
velopment of L. glauca in P- treated Halii and 
Kapaa subsoils in pots is interpreted more as a 
response to P than a decrease in A1 effects. In 
addition, of course, a mass-action effect may be 
operating in which active aluminum in the soil 
is being supplied with enough phosphorus to 
permit complete precipitation as aluminum 
phosphate with enough remaining to supply the 
plant with adequate P. 
Deep tap-root development in the Koolau 
soils was unexpected because of the extremely 
poorly-drained condition of this soil. Root 
growth in the wet Koolau soil in pots also ap- 
peared normal even though figures for extract- 
able A1 were high. 
The lack of root damage in plants used in this 
study was probably related to the evolutionary 
background of the plants, which seem to thrive 
in areas of low fertility and high rainfall. Roots 
of plants sensitive to Al, like rye or barley, 
would probably be severely injured in these 
soils. But Leucaena glauca, like many other 
tropical plants, does not respond markedly to 
lime. 
Thicket formation in R. tomentosa is prob- 
ably caused by the large number of adventi- 
tious shoots which arise from buds on the tap 
root just below the soil surface. 
Heavy staining of roots with hematoxylin 
cannot be definitely attributed to Al alone, since 
iron and other metals may also act as a mordant 
for hematoxylin. However, since in the present 
study chemical analysis showed Al to be present 
in large amounts, much of the staining obtained 
is attributed to Al. The particularly intense 
staining of the outer walls of the epidermis, and 
of two areas in the outer part of the stele, are 
interpreted as an indication that Al precipita- 
tion may have occurred in these areas. 
Our results appear to differ significantly from 
those of McLean and Gilbert (1927) and of 
Wright and Donahue (1953). The reasons for 
these differences are not immediately apparent. 
In all three studies root sections were stained 
in hematoxylin without the addition of a mor- 
dant, and the staining which occurred was inter- 
preted as an indication that aluminum was 
already present in the tissues. 
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