Fixation of Free Nitrogen hy the Lower Ch’een Plants. 429 
sown with nodule-producing microbes. In the course of their 
growth the peas took up the free nitrogen of the air, more than 
half the nitrogen which the plants finally contained being derived 
from this source and the rest from the seed. Peas grown simul- 
taneously in identical soil, which, however, was not sown with 
microbes, fixed no free nitrogen. 
In 1891 the experiments were extended to plants other than 
leguminous species. Plants belonging to several different natural 
orders, as well as peas representing the papilionaceous division of the 
Leguminosje, were grown under similar conditions throughout. 
These diverse plants were : — 
Jerusalem artichoke, — nat. ord. Compositae. 
Oats, — nat. ord. Graminese. 
Tobacco, — nat. ord. Solanaceso. 
Mustard, — nat. ord. Cruciferae. 
Cress „ „ 
Giant spurrey, — nat. ord. Caryophyllaceae. 
Two series of experiments were made in 1891. In the first 
series the surfaces of all the soils became covered, to a greater or less 
extent, with inferior green plants, amongst which were recognised 
certain Mosses (Bryum, Leptobryum) and certain Algae • (Conferva, 
Oscillaria, Nitzschia). 
In all these cases absorption of nitrogen took place, save in two 
instances in which the development of these humble forms of plant- 
life was very feeble. In one case in which none of the higher plants 
were present, the soil became clothed with a notable quantity of the 
lower green plants, and advantage was taken of this circumstance 
to determine what proportion of the absorbed nitrogen had been 
taken up by the plants, and what proportion by the soil. It was 
found that all the nitrogen that had been gained was accounted for 
by the plants, the underlying soil not showing any gain. 
In the second series of the experiments of 1891 measures were 
adopted to prevent the appearance of the inferior green plants. 
This was efl'ected by covering the soil, after the seeds had been 
sown, with a thin layer of calcined quartz-sand, upon which nothing 
' The Algte comprise the lowest form of plant-life. A familiar example 
is afforded by the green material which accumulates in rain-water butts, on 
damp palings and tree-trunks, and in puddles which are drying up ; it con- 
sists of the unicellular plant called Protococcus, which forms an instructive 
object under the microscope. The green threads seen in fresh-water ponds 
afford another common example of an alga ; tliis plant is called Spirogyra, 
and it consists merely of a string of cells placed end to end. The Algse differ 
from the Fungi in this important respect, that although from a structural 
point of view both are very lowly organised, their cells forming no structures 
corresponding with the fibrous tissues of the higher plants, yet the Algae 
produce chlorophyll and can therefore make their own starch, whereas the 
Fungi, containing none of the green pigment chlorophyll, are incapable of a 
function of so much physiological importance. The seaweeds arc Alg®, and in 
red seaweeds the chlorophyll is merely masked by the presence of some other 
pigment. — W. F. 
VOL. in. T. S. — 10 F F 
