THE CAUSES OF THE FERTILITY, ETC. 
43 
Now, the relation of this schizomycete, called bacterium 
radicicola, to the leguminous plant, is an exceedingly 
interesting one. In the beginning the bacterium is free in 
the soil. When it comes in contact with a root-hair of a 
leguminous plant it penetrates the cell-wall of the root-hair, 
which, as you know, consists of a single cell, and multiplies 
rapidly. A colony is thus formed which then sends out 
a tube filled with bacteria — the so-called infection thread — 
which finally pushes its way through the cell-walls of the 
tissues of the root, causing hypertrophy of the cells and 
the formation of a bacteroid tissue. 
It has not been settled yet whether the bacteria give 
rise to the infection thread or whether it is the plant which 
does this. Fischer has justly pointed out that the structure 
of this tube could be formed only from the protoplasm of 
higher plants ; in other words, that the walls of this tube 
consist of a substance allied to cellulose, which is not formed 
by bacteria. Be that as it may, the following facts remain : 
(a) The bacteria enter the root in the first place 
as parasites. If they are weak, the plant 
resists the attack, no nodules are formed, and 
the colonies die out. If the plant is weak, 
then the formation of the nodules is quicker 
and more numerous than in a strong plant. 
( b ) The plant then makes a counter-attack (if 
strong enough) and becomes parasitic on the 
bacteria, abstracting nitrogen from them. 
T'he relation between bacteria and plant is clearly not of 
symbiotic nature, but one of alternate parasitism. 
I am afraid you will think that I have quite abandoned 
my original subject and am lecturing on bacteria. But I 
had to speak at some length on these organisms because I 
wanted to give you an insight into the fundamental prin- 
ciples of the work performed by nitrifying bacteria in soils. 
The knowledge of the few facts that I told you I consider 
the irreducible minimum necessary to understand the 
fertility of our wheat lands correctly. 
Our soils are, especially in the wheat lands, exceedingly 
rich in bacteria, both in number and variety. Our wild 
fiowers partly belong to the family of the leguminosae and 
their roots show numerous nodules caused by bacteria. 
And now I can come back to my original theme. 
The researches made at Rothamstad have shown that 
in Europe nitrification is most active from July to October, 
i.e., while the wheat plant is ripening, and therefore assi- 
milates but little food from the soil, and after it has been 
harvested. The same fact has been proved by a number 
of Agricultural Experimental Stations in North America. 
