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Root-nodules of the Leguminosae. 
it is entirely the reverse. It has, however, been demonstrated generally 
that the addition of a trace of nitrogenous material behaves as a stimulant 
for the rapid production of slime, and Mockeridge found the presence of an 
organic substance, e. g. a humate, had a very marked effect. The acidity 
or alkalinity of the medium also plays a prominent part. Moore found that 
the presence of from 0-33 to 1 per cent, of potassium and sodium salts was 
often sufficient to inhibit the formation of nodules, whilst the same amount 
of calcium and magnesium salts favoured their production. 
It therefore seems probable that the nature of the cell-sap of the root 
will determine not only whether the bacteria shall enter and multiply, but 
also the production of infection threads, which can only be produced in the 
presence and absence of certain definite substances in the medium. This in 
the plant is the cell, and the particular nature of the cell-sap it contains will 
be influenced by that of the soil in which the plant is growing. It is 
consequently of interest that they are absent from the Genisteae type and 
only slightly developed in the Phaseoleae and Trifoleae type, which flourish 
in poor sandy soil, but play an important part in the Viceae and Mimosoideae 
types, which normally inhabit a richer damper soil. In the Genisteae type 
also, where they are absent, the plants grow where there is a deficiency of 
calcium carbonate. There thus seems to be a variation in the morphology 
of the nodule resulting from infection produced primarily by the possible 
response of the bacteria to the cell-sap of the host, and the presence of the 
bacteria induces growth on the part of the host which will naturally be 
influenced by the anatomical peculiarities of that plant. 
Summary. 
1. Plants which produce root-tubercles when infected with the 
nitrogen-fixing organism Bacillus radicicola are of two kinds — legumes 
and non-legumes. 
2 . The root-tubercles of non-legumes are modified lateral roots, but 
those of legumes are exogenous in origin. 
3. In the leguminous nodule the bacteroidal tissue is central and the 
vascular system consists of a number of peripheral strands. 
4. Bacillus radicicola is connected with the assimilation of nitrogen 
from the atmosphere, and although it is polymorphic, cross-inoculation 
occurs. 
5. The leguminous nodules can be placed in four groups : 
I. The Genisteae type, in which the meristem is spherical, the 
vascular supply forms one broad zone across the base, and 
the bacteroidal tissue becomes divided into several parts. 
(a) In Lupinus the primary vascular strand is tetrarch and 
the bacteroidal zones are separated by vascular as 
well as parenchymatous tissue. 
