1893.] Root Tubercles of Leguminose. 785 
infected cells grow and multiply much more rapidly but the 
adjoining cells likewise take on increased activity. On micro- 
scopic examination there will be noted a slight protuberance, 
swelling outward but still covered by the root epidermal layer. 
This incipient tumor consists of two kinds of meristematic 
tissue, the centrally located portion of infected cells and the 
adjoining outer portion of noninfected cells. This meristem 
tissue is engaged in active cell division. The original infected 
cells divide and produce auto-infected daughter cells. Out- 
wardly a corky tissue is developed from a distinct phellogen 
layer, usually one or two cells in thickness. These phellogen 
cells divide tangentially and often contain starch and calcium 
oxalate crystals. I have found a cork layer on all tubercles 
examined thus far. Centrally cell division goes on less rapidly 
until the pericambium of the root is reached which is also 
involved in the change. A vascular bundle starts from the 
root-vascular system and extends peripherally toward the 
infected area until quite near it where it divides into from four 
to seven branches which continue centrifugally (in reference 
to root) through the outer part of the tubercle meristen to near 
its apex. These vascular bundles consists of conducting 
vessels, like those of the root, surrounded by bast cells: the 
whole is encircled by a sheath of cells whose outer walls are 
considerably thickened. 
In spherical tubercles the cambium is of equal thickness in 
all parts. In elongated and irregular forms the meristematic 
tissue is more abundant in the extended points, that is, there 
may be one or more apical areas in the same tubercle. The 
cambium cells are of the ordinary small, angular, closely uni- 
ted, rather thin walled variety. The cells may divide in any 
plane. The cork layer usually consists of thin walled rect- 
angular cells, often with intercellular air passages. In Phaseolus 
vulgaris the cork cells are considerably rounded, in Robinia 
they are rather irregular. The entire cork layer is continuous 
with the unchanged endodermal layer of the root. The entire 
tubercle is covered over by an epidermal layer of cells contig- 
uous with that of the root. No root-hairs are present. Early 
in the development of the tubercle the root-hairs become 
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