195 
Root-nodules of the Leguminosae . 
produced each spring on the new roots and develop through the growing 
season until the autumn, when they decay in the soil. Beijerinck drew atten- 
tion to the perennial nature of the nodules of Robinia. The author has 
examined nodules of Robinia Pse2idacacia> Sophora , and some species of 
Acacia , all of which last for more than one growing period. These plants 
are trees or woody undershrubs which are indigenous in the west temperate 
and sub-tropical regions. The climatic conditions in these parts would 
naturally enable the bacteria to retain their activity for a much longer 
period than in colder regions, and in the nodules of these plants there 
is a copious development of protective tissue from the phellogen, consisting 
of thick-walled cells which are very dark in colour owing to the formation 
of tannin in Robinia and Acacia , but in Sophora they remain much paler, 
no tannin being present. 
These three genera represent three quite different types of Legumi- 
nosae, Acacia being a very large genus in the Mimosoideae, while 
Text-fig. 5. Longitudinal section of nodule and root of Sophora. x 15. x is the first 
bacteroidal zone produced ; z, that produced during the second growing period ; y , the area between 
them, in which the vascular strands anastomose. 
Sophora and Robmia are both members of the Papilionaceae ; but Sophora 
represents the unusual type in which the stamens are free, and Robinia is 
one of the largest trees in the order. They all have characteristic pinnate 
leaves with a large number of pinnules adapted to a greater or less degree 
for movement in response to external stimuli. 
The nodules all develop two vascular strands which have a separate 
attachment to the root-stele, and a well-developed bundle sheath is present. 
In Acacia the meristem extends all round the bacteroidal tissue and its 
activity is renewed from time to time. But the greatest growth always 
takes place so that the nodule becomes bean-shaped (see PI. XIII, Fig. 4). 
In Sophora and Robinia the growing point is apical and the nodule becomes 
elongated and transversely indented. The indentations occur between two 
periods of growth, and consist of cortical cells which in Sophora are trans- 
versed by the vascular strands which anastomose in this area (see Text-fig. 5). 
Infection threads have been observed in connexion with the migration 
of the bacteria into the newer bacteroidal cells. These cells elongate in the 
