190 Spratt. — A Comparative Account of the 
is differentiated from the pericycle, and in this area one or more vascular 
strands appear and become continuous with the vascular tissue of the root. 
By subsequent growth it breaks through the parenchyma of the root and 
may become quite a large structure, consisting of a mass of central cells 
containing bacteria, surrounded by a zone of parenchyma in which there 
are a number of vascular strands, which are either collateral, with the xylem 
towards the outside and exarch, or concentric, with the xylem central. 
Both types may occur in the same nodule. The xylem consists of tracheides, 
usually spirally marked, and parenchyma. The phloem is simply narrow, 
very elongated cells. Each strand is surrounded by a few very definite 
layers of cells which form a kind of bundle-sheath. 
Text-fig. i. a . Longitudinal section of root and young nodule of Abuts, b. Transverse section 
of nodule of Alnus. c. Longitudinal section of part of old branched nodule of Alnus. x 10. 
The nodule is protected by a varying number of layers of cells developed 
from a meristematic layer continuous with the phellogen of the root, which 
may become dead and empty, or in some cases thickened (see PI. XIII, 
Figs. 7 , 8, 9). 
Whatever its ultimate form and however much it may resemble any 
particular non-leguminous nodule, e.g. Vicia Faba and Alnus , the nodules 
in these two classes are fundamentally different. In the non-legume it is 
a modified root, whilst in the legume it is an exogenous growth arising in 
the cortex, which develops a peripheral vascular system and has not a 
growing point which is primarily differentiated in an apical position. 
The leguminous nodules have received a good deal of attention from 
a large number of investigators. In 1867 Woronin described and figured 
them, showing the central mass of bacteroidal cells surrounded by an outer 
layer in which were several vascular bundles. Peirce in 1902 says they are 
modified lateral roots, but subsequently described their origin from the 
layer immediately outside the endodermis. They have, however, been very 
