123 
Alnus and Elaeagnus . 
and a half across. The tubercles are enabled to grow in clusters of such 
dimensions by their perennial habit and the repeated bifurcation which 
occurs when they are from a quarter to half an inch in length and about an 
eighth of an inch in diameter. In some cases, particularly when the tuber- 
cles occur in isolated positions on the roots, trifurcation occurs. 
Transverse sections of the tubercles of both plants (Figs. 3 and 13) 
immediately reveal the fact that they are modified lateral roots, each 
possessing a well-defined central stele, and the increased diameter is caused 
by the enlargement of the cortical cells owing to their infection with the 
Bacteria. The organisms penetrated the root-hair, and entering the cortical 
cells caused them to hypertrophy, and this arrested the growth in length of 
the stele, so that a short swollen structure is produced instead of a typical 
lateral root. The tip of the nodule is occupied by a meristematic zone 
(Figs. 7 and 16), by means of which it grows, not only during one season, 
but also from year to year, growth being renewed each spring, when both 
meristematic apex and central stele branch (Figs. 7 and 16). The vascular 
tissue in the tubercles of Elaeagnus is in the form of a triarch stele (Fig. 13), 
possessing a relatively large amount of phloem, two or three layers of 
pericycle, and a distinct endodermis which is always closely packed with 
reserve food material, usually in the form of oil, but in some of the tubercles 
gathered in the spring there was a large proportion of starch. The xylem 
is composed of annular and spiral vessels and parenchyma. Towards the 
base of the older nodules a considerable amount of secondary vascular 
tissue is produced, and amongst this xylem there are scalariform vessels. In 
the tubercles of Alnus the central stele varies from triarch to hexarch 
(Figs. 3, 4, and 5). In the older regions there is a solid central mass 
of xylem (Fig. 5), whilst at the tip the centre of the stele is occupied 
by parenchyma, and the protoxylems are arranged round this (Fig. 4). 
Here also secondary thickening occurs, a considerable quantity of phloem 
is present, surrounded by two or three layers of pericycle and an endo- 
dermis, the cells of which are filled with reserve food material in the 
form of definite structures usually somewhat spherical in shape and 
giving very definite proteid reactions. 
In the roots the phellogen has its origin in the pericycle, and forms 
a few layers of periderm externally, and several layers of secondary cortex 
on the inside. The tubercles are always surrounded by a few layers of 
periderm (Figs. 3, 7, 13, 16), which are formed by the continued growth of 
the original phellogen of the root to keep pace with the growth of the 
tubercle. In the spring the periderm near the tips of the tubercles in 
Elaeagnus frequently becomes very irregular, large masses being split off at 
various places, giving rise to small excrescences on the surface which are 
sometimes visible to the naked eye. This is caused by the inability of 
the phellogen to divide sufficiently rapidly to keep pace with the 
