Root-nodules of the Leguminosae. 193 
active than in nodules of other genera, since its development causes the 
separation of the bacteroidal tissue into several zones. 
This group may be subdivided thus : 
{a) In Lupinus , Ornithopus , Cytisus , and Desmodium the primary 
vascular strand is tetrarch and divides into four, correlated with which 
four growing points are differentiated. The bacteroidal zones become 
separated by a broad band of parenchyma, in the centre of which a vascular 
strand appears (see Text-fig. 2). 
(b) In Genista , Ulex , Spartium , and. Amorpha the primary vascular 
strand at the base is diarch and 
divides into two strands which 
traverse the nodule some distance 
before branching again. The 
bacteroidal zones are separated 
by a narrow band of parenchy- 
matous cells. In the old nodule 
of Ulex an apical meristem is 
differentiated, but more usually 
there is one to each bacteroidal 
zone (see Text-fig. 3). 
II. The Phaseoleae and Tri- 
foleaetype ofnodule are very simi- 
lar to those above (PI. XIII, Fig. 2), but the bacteroidal tissue always remains 
an undivided central zone. The growing point at an early stage frequently 
becomes localized apically ; consequently they elongate, although usually 
Text-fig. 3. Longitudinal section of nodule and 
root of Spartium . x 20. 
Text-fig. 4. a. Longitudinal section of nodule and root of Phaseolus. x 10. b . Longitudinal 
section of old nodule and root of Lotus corniculatus . x 15. 
remaining very narrow, e. g. Trifolium , and frequently the apical meristem 
branches so that a repeatedly branched nodule may result, e. g. Lotus 
corniculatus (see Text-fig. 4 and PI. XIII, Fig. 3). 
There is greater development of infection threads than in the previous 
groups. Dawson described them as being present only in the young 
