126 Spratt . — The Morphology of the Root Tubercles of 
what compressed laterally, and here and there are little colonies of Bacteria 
(Fig. 15) which have not migrated to the younger tissue nearer the tip of 
the tubercle, although they assume the rod-shaped form in the spring. 
The cortical cells of the tubercles of Alnus also clearly contain organ- 
isms of two forms — small bacilli, undoubted Pseudomonas radicicola , and 
larger coccus forms (Fig. 6). These forms are evidently analogous to the 
same forms in Elaeagnus. Cells are frequently met with also, which con- 
tain a mixture of these two forms in varying proportions (Fig. 6), and 
in some tubercles the spherical bodies are seen to be in a state of division, 
so that they clearly produce rods again (Fig. 6, d). Alnus tubercles were 
subjected to the cultural methods described above for Elaeagnus, which 
revealed the same polymorphic nature of the organism inhabiting it, the 
only difference being that the Alnus bacillus does not form such an evident 
zoogloea in the cells of its host. Probably correlated with this, the nuclei of 
the cortical cells do not appear to undergo any definite changes. In Alnus , 
too, the bacteroidal cells traverse the complete length of the nodule, and be- 
tween them, in zones more or less concentric with the endodermis, are cells 
containing chiefly reserve food material in the form of oil and proteid globules. 
The entire absence in all the tubercles examined of any filaments 
or hyphae in Alnus , and only the very narrow zoogloea threads being present 
in Elaeagnus , together with the inability of the spherical bodies, which 
appear to have been called ‘ spores * by some authors, to germinate after the 
manner of a spore, seems to preclude the possibility of the tubercles being 
produced by the parasitism of a spore-producing hyphal-fungus. The 
present series of investigations entirely support the idea that the root 
tubercles of Alnus and Elaeagnus , although morphologically different, are 
physiologically analogous to the root tubercles of the Leguminoseae, which 
have been shown to be actively concerned in the fixation of atmospheric 
nitrogen, thus rendering this vast store available to the leguminous plant. 
This is done for Alnus and Elaeagnus by a species of Pseudomonas radici- 
cola, which, in one of its forms, is morphologically identical with the 
organism found in leguminous nodules, but like the latter it is polymorphic 
and assumes a comparatively large spherical form, which does not usually 
occur in other species. 
In conclusion, my heartiest thanks and gratitude are due to Professor 
W. B. Bottomley, in whose laboratory the investigations have been pursued, 
for his kindness, sympathy, and advice. 
Summary. 
1. The root tubercles of Alnus and Elaeagnus are modified lateral 
roots. They are perennial, dichotomously or trichotomously branched 
structures. 
