1893.] Root Tubercles of Leguminose. 783 
etiologically resemble those growths produced by certain 
animals as for example the Heterodora or Anguillula cysts found 
on some roots. Morphologically, however, they are clearly 
distinct. 
In some cases these tubercles make their appearance shortly 
after germination. In others not until plants have reached 
the flowering stage or even later. In some cases they do not 
appear at all, that depending upon the nature of soil in which 
the plants grow as explained by Frank? and others. The 
macroscopic appearances of some tubercles have been more or 
less perfectly described in a previous article! The form is 
quite constant in a given species of plant. In Phaseolus 
vulgaris and Amphicarpaea comosa, for instance, they are spheri- 
cal In Melilotus alba, Pisum sativum, Trifolium pratense and 
Trifolium repens they are oblong, narrow at point of attachment, 
often slightly forked, cordate or pear shaped. Usually they 
occur singly, less often, as for example in Melilotus alba they 
grow into large grape-like bunches. Like root branches they 
usually develop acropetally though there are numerous excep- 
tions. They are most common on roots near the surface of the 
soil ; sometimes they are found at a considerable depth. They 
are never found in close proximity to growing roottips. They 
occur either on the main root or its branches. 
Tubercles seem always to develop exogenously. The direct 
causes of their development are the Rhizobia, of which there 
are at least several species? Predisposing causes are, nature of 
soil and condition of atmosphere as to temperature and mois- 
ture, etc. The various kinds of Rhizobia appear to be more 
or less abundant in all normal soil. Whenever a given species 
finds a suitable lodging place on the root-surface it multiplies 
and by some means gains access to one or more of the surface 
cells and infection is completed. Just how infection takes 
place is not definitely known. It is thought that the Rhizobia 
have the power to dissolve the cell membrane of the host; 
just how is not stated. Frank, Moeller’ and others maintain 
3 Pilz symbiosa der Leguminosen, 1890. 
* Bulletin Torrey Bot. Club, July, 1892. 
5 Bulletin Torrey Bot. Club, July, 1892. 
ê Ber. der deutsch. bot. Ges., Heft 5, 1892. 
