TUBERCULAR SWELLINGS ON THE ROOTS OF VICIA FABA. 
555 
tubercles occur on water-cultures, and I have now shown that most satisfactory 
results can be obtained by means of water-cultures. Tschirch also states that they 
are commoner on the roots of plants growing in poor soil (as regards nitrogen) than 
on those in rich humus-soils, a statement I fail to confirm. Darkness, drought, and 
other disturbing influences of the kind hinder the proper development of the 
tubercles ; and the rule is that the tubercles flourish in proportion to the thriving 
of the plant. 
It is obvious, on a little reflection, that there would be nothing in these facts 
which contradicts the explanation I have given. Since the gemmules stimulate the 
protoplasm of the cells to greater activity—as indicated by the increased supplies of 
starch and the hypertrophied growth—it is clear that the needs for the increased 
activity will be satisfied only in proportion to the ability of the host-plant to furnish 
supplies : darkness would cut off the supplies of carbo-hydrates, and drought those 
of water and minerals, and thence the parasite and protoplasm in the tubercles suffer. 
This can be directly shown in water-cultures, as well as in the case of plants growing 
in the open. As regards the generalisation that the tubercles are commoner in soils 
which are poor in nitrogenous substances, this might be because there is more oxygen 
supplied to the very large active root-systems in such soils ; a view, moreover, which 
is in accordance with the well-known fact that the tubercles are apt to be more 
abundant and larger on those parts of the roots which are nearer the surface of the 
soil, though this is not necessarily the case. There is another point to note here. 
Tschirch himself observes (p. 90) that the Leguminosse are deep-rooted plants which 
do not thrive well in rich humus-soils, abounding in nitrogenous materials. Now, 
without agreeing with him to the extent of his generalisations (for it is by no means 
established that the Leguminosse as a class are dependent on one kind of soil), it is 
obvious that nothing in this contradicts the explanation of the tubercles as hyper¬ 
trophies due to the stimulating action of a symbiotic Fungus. Tschirch lays great 
stress on the facts that the analysis of the tubercles shows them to be relatively very 
rich in potassium, phosphorus, and nitrogen, and claims that the abundance of nitrogen 
is due to the “ bacteroids no doubt this is the case, since the “ bacteroids ’’are of the 
nature of Fungi—yeast-cells—structures known to be particularly rich in potassium and 
phosphorus as well as nitrogen.* Tschirch employs the evidence of the analysis 
in support of the hypothesis that the tubercles are normal organs used as storehouses 
for reserve materials : the evidence, however, distinctly supports the view that the 
protoplasm becomes surcharged with materials employed in the hypertrophy and for 
the good of the parasite. Other facts, quoted by Tschirch in a different sense, are 
equally in favour of this : for instance, the absence of sugar and the presence of fatty 
substances in the winter. 
At the end of his memoir Tschirch gives a summary substantially as follows:— 
The Leguminosese are plants which require much nitrogen, but only towards the 
* Cf. Husemann, ‘ Die Pflanzenstoffe, 5 1882, vol. 1, p. 279. 
4 B 2 
