CHAPTER VI._The micro-organisms of the soil

ALGÆ.

I. GENERAL AND HISTORICAL INTRODUCTION.

Speaking broadly, the organisms of the soil may be classified into

several distinct groups differing conspicuously in their general

characters and physiological functions and therefore in their economic

significance; among such groups may be mentioned the bacteria,

protozoa, algæ and fungi. It is found, however, that though typical

members of these groups are conspicuously different from one another,

yet there exist a number of unicellular forms which have characters

in common with more than one of these big groups, and the lines of

demarcation between them become difficult to define. It becomes

advisable, therefore, to depart a little from the systematist’s rigid

definitions and to adopt a somewhat more logical grouping of the soil

organisms based on their mode of life.

To give but a single example: _Euglena viridis_ occurs quite commonly

in soil. Through its single flagellum, its lack of a definite cellulose

wall, its changeable shape and its ability to multiply by simple

fission in the motile state it definitely belongs systematically to the

group of protozoa. But its possession of chlorophyll, in enabling it to

synthesise complex organic substances from CO₂ and water in a manner

entirely typical of plants, connects it physiologically so closely with

the lower green algæ that in studying the biology of the soil it seems

best to include it and other nearly related forms with the algæ.

On this physiological basis “soil-algæ” may be defined as those

micro-organisms of the soil which have the power, under suitable

conditions, to produce chlorophyll. Such a definition has the

advantage that it is wide enough to include the filamentous protonema

of mosses, which, though alga-like in form and in physiological

action, is nevertheless separated from the true algæ by a wide gulf.

A more accurate name for such a group of organisms would be the

“chlorophyll-bearing protophyta” of the soil; they may be classified

briefly as follows (Table IX.):--

TABLE IX.

+----+------------------------------------+-----------+--------------+

+----+------------------+-----------------+-----------+--------------+

| | | | | |

| II.|_Algæ_-- | | | |

| | | etc. | | |

| | | | | |

+----+------------------+-----------------+-----------+--------------+

The importance of the lower algæ from a biological standpoint has

long been recognised, since their extremely primitive organisation,

coupled with their ability to synthesise organic compounds from simple

inorganic substances, singles them out as being not very distantly

removed from the group of organisms in which life originated upon the

earth. But the possibility of their having a very much wider economic

significance was completely overlooked until about a quarter of a

century ago, when Hensen demonstrated their importance in marine

plankton as the producers of the organic substance upon which the

whole of the animal life of the ocean is ultimately dependent. In

consequence, it has been generally assumed that the growth of algæ,

since they contain chlorophyll, is entirely dependent on the action of

light. Hence the recent idea of the existence of algæ which actually

inhabit the soil has been received with a certain amount of scepticism,

though the results of modern physiological research on a number of the

lower algæ show that there is very good reason to believe that such a

soil flora is entirely possible.

In considering the alga-flora of a soil it is necessary to distinguish

between two very different sets of conditions under which the organisms

may be growing. In the first place, they may grow on the surface of the

soil, being subjected directly to insolation, rain, the deposition of

dew, the drying action of wind, relatively quick changes of temperature

and other effects of climate. Certain combinations of these conditions

present so favourable an environment for the growth of algæ that at

times there appears on the surface of the soil a conspicuous green

stratum, sometimes so dark in colour as to appear almost black. Strata

of this nature are well known, and in systematic works there are

constant references to species growing “on damp soil”; for instance, of

the 51 well-defined species of _Nostoc_ recognised by Forte, no less

than 31 are characterised as terrestrial. Such appearances, however,

seem to have been regarded as sporadic and more or less accidental,

rather than as the unusually luxuriant development of an endemic

population, and have been frequently attributed to an excessively moist

condition of the soil due to defective drainage.

In the second place, the algæ may be living within the soil itself,

away from the action of sunlight and under somewhat more uniform

conditions of moisture and temperature.

Up to the present time the greater number of the investigations carried

out in this subject have been of a systematic nature, and extremely

little direct evidence has been obtained which can throw any light on

the subject of the economic significance of the soil algæ.

The earliest systematic work was carried out by Esmarch, in 1910-11,

who investigated by means of cultures the blue-green algæ of a number

of soils from the German African Colonies, the samples being taken from

the surface and also from the lower layers of the soil. He obtained a

considerable number of species and observed that in cultivated soils

they were not confined to the surface but occurred regularly to a depth

of 10-25 cms. and occasionally as low as 40-50 cms. He attributed

their existence in the lower layers to the presence of resting spores

carried down in the processes of cultivation, since his samples from

uncultivated soils were unproductive.

Later, Esmarch extended his investigations to a far larger

number of samples, 395 in all, of soils of different types from

Schleswig-Holstein. He found that blue-green algæ were very widely

distributed in soils of certain types, though they occurred rarely

in uncultivated soils of low water-content, and he described no less

than 45 species of which 34 belonged to the _Oscillatoriaceæ_ and

_Nostocaceæ_. Certain of the commoner species were obtained from soils

of widely different types, as shown in Table X., while other forms

occurred only rarely and with a much more limited distribution.

TABLE X.--FREQUENCY OF OCCURRENCE OF CERTAIN COMMON SPECIES IN

ESMARCH’S SOIL SAMPLES.

+-------------------------+----------------------------------------+

| | Percentage of Samples |

| | containing given Alga. |

| +--------------------+-------------------+

| | Uncultivated | Cultivated |

| | Damp Sandy Soil. | Soils. |

| +------+------+------+------+-----+------+

| | of | of | Sea- | | |Marsh-|

+-------------------------+------+------+------+------+-----+------+

|Anabæna variabilis | 46 | 43 | 9 | 10·3 | 60 | 46 |

|Anabæna torulosa | 31 | 14·3 | 63·6 | 27·6 | 34·3| 56·4 |

|Cylindrospermum muscicola| 23 | 28·6 | 0 | 24 | 48·6| 59 |

|Cylindrospermum majus | 0 | 14·3 | 0 | 38 | 40 | 33·3 |

|Nostoc Sp. III. | 7·7 | 0 | 0 | 38 | 37 | 48·7 |

+-------------------------+------+------+------+------+-----+------+

Taking the number of samples containing blue-green algæ as a rough

measure of their relative abundance, Esmarch obtained the following

interesting figures (Table XI.):--

TABLE XI.

+------------------------------+----------------+---------+

| | Percentage | |

| | of Samples |Number of|

| | Containing | Samples |

| Kind of Soil. |Blue-green Algæ.|Examined.|

+------------------------------+----------------+---------+

| | | |

|Cultivated marshland | 95 | 40 |

|Cultivated clay soil | 94·6 | 37 |

|Uncultivated moist sandy soils| 88·6 | 35 |

|Cultivated sandy soil | 64·4 | 45 |

| {Woodland | 12·5 | 40 |

|Uncultivated{Sandy heathland | 9 | 34 |

| {Moorland | 0 | 35 |

+------------------------------+----------------+---------+

In noting that the soils fell into two groups, those relatively rich

and those poor in blue-green algæ, Esmarch concluded that the two

chief factors governing the distribution of the _Cyanophyceæ_ on the

surface of soils are, (1) the moisture content of the soil, (2) the

availability of mineral salts, cultivated soils being especially

favoured in both of these respects. He further distinguished between

cultivated land of two kinds, viz. arable land and grass land, and

found that on all types of soil grassland was richer in species than

was arable land.

Esmarch examined, in addition, 129 samples taken from the lower layers

of the soil immediately beneath certain of his surface samples, 107 at

10-25 cms. and the rest at 30-50 cms. depth.

In cultivated soils, whether grassland or arable land, he found that

blue-green algæ occurred almost invariably in the lower layers in those

places bearing algæ on the surface and that, with rare exceptions, the

algæ found in the lower layers corresponded exactly to those on the

surface, except that with increasing depth there was a progressive

reduction in the number of species.

In uncultivated, moist, sandy soils the agreement was far less

complete, for though algæ were rarely absent from the lower layers

their vertical distribution was frequently disturbed by the action

of wind and rain. Other uncultivated soils not subject to periodic

disturbance were found to be uniformly lacking in algæ in the lower

layers, but as the limited number of samples examined came completely

from places where there were no algæ on the surface this means very

little.

By direct microscopic examination of soil Esmarch claims to have

found living filaments of blue-green algæ at various depths below the

surface. He realised, however, that there was no indication of the

length of time that such filaments had been buried, and therefore

conducted a series of experiments from which he concluded that the

period during which the algæ investigated could continue vegetatively

in the soil after burial varied with different species from 5-12 weeks,

but that during the later part of the period the algæ gradually assumed

a yellowish-green colour.

It is unfortunate that Esmarch’s investigations were directed only

towards the blue-green algæ since observations made in this country

indicate that such a series of records gives but a very incomplete

picture of the soil flora as a whole.

Petersen, in his “Danske Aërofile Alger” (1915) added considerably to

our knowledge of soil algæ, especially of diatoms. Unfortunately he

confined his investigations of the green algæ to forms growing visibly

on the surface of the ground. He observed, however, that acid soils

possessed a different flora from that commonly found on alkaline or

neutral soils, the former being dominated by _Mesotænium violascens_,

_Zygnema ericetorum_, and 2 spp. of _Coccomyxa_, while the latter were

characterised by _Mesotænium macrococcum var._, _Hormidium_, 2 spp.,

and _Vaucheria_, 3 spp.

Of diatoms he obtained no less than 24 species and varieties from

arable and garden soils, and five characteristic of marshy soils, while

from forest soils and dry heathland they appeared to be often absent.

He omitted all reference to blue-green algæ.

Meanwhile Robbins, examining a number of Colorado soils that contained

unprecedented quantities of nitrate, obtained from them 18 species of

blue-green algæ, 2 species of green algæ, and one diatom. Moore and

Karrer have demonstrated the existence of a subterranean alga-flora of

which _Protoderma viride_, the most constantly occurring species, was

shown to multiply when buried to a depth of one metre.

In this country attention was first called to the subject by Goodey and

Hutchinson of Rothamsted who, in examining certain old stored soils

for protozoa, obtained also a number of blue-green forms which were

submitted to Professor West for identification. This ability of certain

algal spores to retain their vitality for a long resting period was so

very striking that an investigation was begun at Birmingham in 1915 to

ascertain whether other forms were equally resistant. The investigation

was carried out on a large number of freshly collected samples of

arable and garden soils which were first aseptically air-dried for at

least a month and then grown in culture. No less than 20 species or

varieties of diatoms, 24 species of blue-green and 20 species of green

algæ were obtained from these cultures (Table XII.). In the majority

of the samples there was found a central group of algæ, including

_Hantzschia amphioxys_, _Trochiscia aspera_, _Chlorococcum humicola_,

_Bumilleria exilis_ and rather less frequently _Ulothrix subtilis

var. variabilis_, while moss protonema was universally present. These

species were thought to form the basis of an extensive ecological plant

formation in which, by the inclusion of other typically terrestrial

but less widely distributed species smaller plant-associations were

recognised.

In certain of the soils, associations consisting very largely of

diatoms were present, and it is to be noted that the majority of the

forms that have been described are of exceedingly small size. It is

doubtless this characteristic which enables them to withstand the

conditions of drought to which the organisms of the soil are liable to

be subjected, small organisms having been shown to be better able to

resist desiccation than are larger ones. Since the soil diatoms belong

to the pennate type, they are further adapted to their mode of life by

their power of locomotion, which enables them in times of drought to

retire to the moister layers of the soil.

In the soils examined in this work blue-green algæ were less

universally present than were diatoms or green algæ, and the species

found appeared to be more local in occurrence. There seemed to be,

however, an association between the three species, _Phormidium tenue_,

_Ph. autumnale_, and _Plectonema Battersii_, at least two of the three

species having been found together in no less than 16 of the samples,

while all three occurred in 7 of them.

TABLE XII.--ALGÆ IN DESICCATED ENGLISH SOILS. (BRISTOL.)

+---------------+-----------+------------------------------+

| | | Number of Species. |

| | Number of +-----------+-----------+------+

+---------------+-----------+-----------+-----------+------+

|Diatoms | 95·5 | 9 | 3·7 | 20 |

|Blue-green algæ| 77·3 | 7 | 2·5 | 24 |

|Green algæ | 100 | 7 | 4·3 | 20 |

|Moss protonema | 100 | -- | -- | -- |

+---------------+-----------+-----------+-----------+------+

| Total | -- | 20 | 10·5 | -- |

+---------------+-----------+-----------+-----------+------+

It was generally noticeable that those soils found to be rich in

blue-green algæ contained only a few species of diatoms, and vice

versa. Diatoms appeared most frequently in soils from old gardens,

whereas blue-green algæ were more characteristic of arable soils. The

green algæ and moss protonema, on the other hand, were distributed

universally.

The majority of green algæ typically found in soils are unicellular,

but a few filamentous forms occur. With the exception of _Vaucheria_

spp. these are characterised, however, by an ability to break down

in certain circumstances into unicellular or few-celled fragments, in

which condition identification is often very difficult.

It was also found by cultural examination of a number of old stored

soils from Rothamsted that germination of the resting forms of a

number of algæ could take place after an exceedingly long period of

quiescence. No less than nine species of blue-green algæ, four species

of green algæ, and one species of diatom were obtained from soils that

had been stored for periods of about forty years, the species with the

greatest power to retain their vitality being _Nostoc muscorum_ and

_Nodularia Harveyana_.

II. THE SOIL AS A SUITABLE MEDIUM FOR ALGAL GROWTH.

Were it not for the recent advances that have been made in our

knowledge of the mode of nutrition of many of the lower algæ, it would

be very difficult to account for the widespread occurrence of algæ in

the soil, for it is undoubtedly true of some of the more highly evolved

algæ that their mode of nutrition is entirely typical of that of green

plants in general. The application of bacteriological technique to the

algæ, however, by Beijerinck, by Artari, and by Chodat and his pupils,

and the introduction of pure-culture methods have led to a study of

the physiology of some of the lower algæ, in the hope of getting to

understand some of the fundamental problems underlying the nutrition

of organisms containing chlorophyll. It is impossible here to do more

than mention the names of a few of the more important of those who

have worked along these lines, such as Chodat, Artari, Grintzesco,

Pringsheim, Kufferath, Nakano, Boresch, Magnus and Schindler, and to

condense into a few sentences some of their more important conclusions.

It is now established that although in the light the algæ are able to

build up their substance from CO₂ and water containing dilute mineral

salts, yet in such conditions growth is sometimes very slow, and with

some species at any rate it is greatly accelerated by the addition of

a small quantity of certain organic compounds. The ability of the lower

algæ to use organic food materials varies specifically, quite closely

related forms often reacting very differently to the same substance,

but there have been shown to be a considerable number of forms which

can make use of organic compounds to such an extent that they can

grow entirely independently of light. In such cases the nutrition

of the organism becomes wholly saprophytic, and the chlorophyll may

be completely lost; it has frequently been observed, however, that

on suitable nutrient media, even in complete darkness, certain algæ

continue to grow and retain their green colour, provided that a

sufficient supply of a suitable nitrogenous compound is present.

_Chlorella vulgaris_, an alga frequently found in soil, has been shown

to be extremely plastic in its relations to food substances. Given only

a dilute mineral-salts solution as food source, it absorbs CO₂ from

the air, and grows in sunlight with moderate rapidity. The addition

of glucose to the medium in the light greatly increases the rate and

amount of growth and the size of the cells, while in the dark the

colonies not only remain green but have been shown to develop more

vigorously than in full daylight. The organism is also able to use

peptone as a source of nitrogen in place of nitrates.

_Stichococcus bacillaris_ and _Scenedesmus spp._, also occurring

in soils, have been shown to be almost equally adaptable, though

in these cases the organisms grow more slowly in the dark than on

the corresponding medium in the light. Liquefaction of gelatine by

the secretion of proteolytic enzymes has been shown to be a further

property of certain species, resulting in the formation of amino acids

such as glycocoll, phenylalanine, dipeptides, etc. This property is,

however, possessed by only a limited number of species and in varying

degree.

Up to the present very little work of this kind has been done upon algæ

actually taken from the soil, and our knowledge is therefore very

scanty. Of the species so far examined all show considerable increase

in growth on the addition to the medium of glucose and other sugars,

and tend to be partially saprophytic; a few have been shown to liquefy

gelatine to some extent.

Servettaz, Von Ubisch, and Robbins have also demonstrated that the

protonema of some mosses can make use of certain organic substances,

especially the sugars, and grow vigorously in the dark. It has been

shown, however, that light is essential for the development of the moss

plant.

It was thought at Rothamsted that some light might be thrown upon the

activities of the soil-algæ by making counts of the numbers present

in samples of soil taken periodically within a circumscribed area.

A dilution method similar to that in use in the protozoological

laboratory was adopted and applied to samples of arable soil taken

from the surface, and at depths of 2, 4, 6 and 12 inches vertically

beneath. A considerable number of samples were examined in this way

from two plots on Broadbalk wheat-field, viz.: the unmanured plot and

that receiving a heavy annual dressing of farmyard manure. The numbers

in the unmanured soil were observed to fall far short of those in that

containing a large amount of organic matter, while in both plots the

numbers varied considerably at different times of the year. The chief

species in both plots were identical, and their vertical distribution

was fairly uniform, but it was observed that the numbers of individuals

varied according to the depth of the sample. The 6th and 12th inch

samples contained very few individuals of comparatively few species,

but the 4th inch samples yielded numbers that were not significantly

less than those in the top inch. The 2nd inch sample was usually much

poorer in individuals than either the top or the 4th inch.

It is unfortunate that this method of counting is not really

satisfactory for the algæ, chiefly because it takes no account of the

blue-green forms. The gelatinous envelope which encloses the filaments

of these algæ prevents their breaking up into measurable units.

Assuming, as appears to be the case for the two plots investigated,

that the blue-green algæ are at least as numerous as the green forms,

the total numbers should probably be at least twice as great as those

calculated. Taking 100,000 as a rough estimate of the number of algæ

per gram of manured soil in a given sample, and assuming the cells to

be spherical and of average diameter 10µ, it has been calculated that

the volume of algal protoplasm present was at least 3 times that of

the bacteria though only one-third of that of the protozoa. This is

probably only a minimum figure for this sample.

A soil population of this magnitude can not be without effect on the

fertility of the soil. When growing on the surface of the ground

exposed to sunlight the algæ must, by photosynthesis, add considerably

to the organic matter of the soil, but when they live within the soil

itself their nutrition must be wholly saprophytic, and they can be

adding nothing either to the energy or to the food-content of the soil.

How these organisms fit into the general scheme of life in the soil is

at present undetermined, and there is a wide field for research in this

direction.

III. RELATION OF ALGÆ TO THE NITROGEN CYCLE

Probably the most important limiting factor in British agriculture is

the supply of nitrogen available for the growing crop, and it seems

likely that the soil-algæ are intimately connected with this question

in several ways.

Periodic efforts have been made during the last half century to

establish the fact that a number of the lower organisms, including

the green algæ, have the power of fixing atmospheric nitrogen and

converting it into compounds which are then available for higher

plants. This property has been definitely established for certain

bacteria, and rather doubtfully for some of the fungi, but until

recently no authentic proof had been produced that algæ by themselves

could fix nitrogen. The subject is too wide to be discussed in much

detail here.

Schramm in America, working with pure cultures of algæ, tried for

ten years to establish the fact of nitrogen fixation, and failed

completely; more recently Wann has extended Schramm’s work, and claims

to have proved indisputably that, given media containing nitrates as

a source of nitrogen and a small amount of glucose, the seven species

of algæ tested by him fixed atmospheric nitrogen to the extent of 4-54

per cent. of the original nitrogen content of the medium. So important

a result needed corroboration, and Wann’s experiment, with some slight

improvements, was therefore repeated at Rothamsted last summer.

This work has not yet been published, but in the whole series of

ninety-six cultures, with four different species, each growing on

six different media, there is no evidence that nitrogen fixation has

taken place; but there has been a total recovery at the end of the

experiment of 98·93 per cent. of the original nitrogen supplied. On

the other hand, a flaw has been detected in Wann’s method of analysing

those media containing nitrates, sufficiently great to account for the

differences he obtained between the initial and final nitrogen content

of his cultures. Hence, though one hesitates to say that the algæ are

unable, given suitable conditions, to fix atmospheric nitrogen, one

must admit that no one has yet proved that they can do so.

It is far more likely, however, that the experiments of Kossowitsch

and others throw more light on the relation of soil algæ to

nitrogen fixation. They affirm that greater fixation of nitrogen is

effected by mixtures of bacteria and certain gelatinous algæ than

by nitrogen-fixing bacteria alone, and that the addition of algæ to

cultures of bacteria produces a stimulating effect only slightly less

than that of sugar. It is probable, therefore, that the algæ, in their

gelatinous sheaths, provide easily available carbohydrates from which

the bacteria derive the energy essential to their work, and that

nitrogen fixation in nature is due to the combined working of a number

of different organisms rather than to the individual action of single

species.

Russell and Richards have shown that the rate of loss of nitrogen by

leaching from uncropped soils is far less than would be expected from

a purely chemical standpoint, and suggest that certain organisms are

present in the soil which, by absorbing nitrates and ammonium salts

as they are formed, remove them from the soil solution and so help to

conserve the nitrogen of the soil. It is probable that the soil algæ

act in this manner, though to what extent has not yet been determined.

IV. RELATION OF ALGÆ TO SOIL MOISTURE AND TO THE FORMATION OF HUMUS

SUBSTANCES.

In warmer countries than our own, especially those with an adequate

rainfall, the significance of soil algæ is perhaps more obvious to

a casual observer. Treub states that after the complete destruction

of the island of Krakatoa by volcanic eruption in 1883, the first

colonists to take possession of the island were six species of

blue-green algæ, viz., _Tolypothrix_ sp., _Anabæna_ sp., _Symploca_

sp., _Lyngbya_ 3 spp. Three years after the eruption these organisms

were observed to form an almost continuous gelatinous and hygroscopic

layer over the surface of the cinders and stones constituting the soil,

and by their death and decay they rapidly prepared it for the growth

of seeds brought to the island by visiting birds. Hence the new flora

which soon established itself upon the island can be said to have

had its origin in the alga-flora which preceded it. Fritsch has also

emphasised the importance of algæ in the colonisation of new ground in

Ceylon.

Welwitsch ascribes the characteristic colour from which the “pedras

negras” in Angola derive their name to the growth of a thick stratum of

_Scytonema myochrous_, a blue-green alga, which gradually becomes black

and completely covers the soil. At the close of the rainy season this

gelatinous stratum dries up very slowly, enabling the underlying soil

to retain its moisture for a longer period than would otherwise be the

case.

The gelatinous soil algæ are probably very important in this respect,

for their slow rate of loss of water is coupled with a capacity for

rapid absorption, and they are therefore able to take full advantage of

the dew that may be deposited upon them and increase the power of the

soil to retain moisture.

V. RELATION OF ALGÆ TO GASEOUS INTERCHANGES IN THE SOIL.

In the cultivation of rice the algæ of the paddy field have been found

to be of extreme importance. Brizi in Italy has shown that although

rice is grown under swamp conditions yet the roots of the rice plant

are typical of those of ordinary terrestrial plants and have none

of the structural adaptations to aquatic life so characteristic of

ordinary marsh plants. Hence the plants are entirely dependent for

healthy growth upon an adequate supply of oxygen to their roots from

the medium in which they are growing. A serious disease of the rice

plant, characterised by the browning and dying off of the leaves,

which was thought at first to be due to the attacks of fungi, was

found to be the effect of the inadequate aeration of the roots, while

the entry of the fungi was shown to be subsequent to the appearance

of the physiological disease. The presence of algæ in the swamp water

was found to prevent the appearance of this disease, in that they

unite with other organisms to form a more or less continuous stratum

over the surface of the ground, and add to the gases which accumulate

there large quantities of oxygen evolved during photosynthesis. The

concentration of dissolved oxygen in the water percolating through the

soil is thereby raised to a maximum, and the healthy growth of the crop

ensured.

This work has been corroborated by Harrison and Aiyer in India, and a

sufficient supply of algæ in the swamp water is now regarded as one of

the essentials for the production of a good rice crop.

From what has been said, it appears that, although our knowledge of

the soil algæ is extremely limited, and our conception of the part

they play is largely based on speculation, yet the subject is one of

enormous interest and worthy of investigation in many directions. In

its present undeveloped state, it is a little difficult to foresee

which lines of study are likely to prove most profitable, but there

is little doubt that eventually the soil algæ will be shown to play a

significant part in the economy of the soil.

SELECTED BIBLIOGRAPHY.

* _Papers giving extensive bibliographies._

I. GENERAL.

[1] Bristol, B. M., “On the Retention of Vitality by Algæ from Old

Stored Soils,” New Phyt., 1919, xviii., Nos. 3 and 4.

[2] Bristol, B. M., “On the Alga-Flora of some Desiccated English

Soils: an Important Factor in Soil Biology,” Annals of Botany, 1920,

vol. xxxiv., No. 133.

[3] Brizi, U., “Ricerche sulla Malattia del Riso detta ‘Brusone,’

Sect. IV. Influenza che le alghe verdi esercitano in risaia,”

Annuario dell Instituzione Agraria Dott. A. Ponti, Milan, 1905, vol.

vi., pp. 84-89.

[4] Esmarch, F., “Beitrag zur Cyanophyceen-Flora unserer Kolonien,”

Jahrb. der Hamburgischen wissensch. Anstalten, 1910, xxviii., 3.

Beiheft, S. 62-82.

[5] Esmarch, F., “Untersuchungen über die Verbreitung der

Cyanophyceen auf und in verschiedenen Boden,” Hedwigia, 1914, Band

lv., Heft 4-5.

[6] Fritsch, F. E., “The Rôle of Algal Growth in the Colonisation of

New Ground and in the Determination of Scenery,” Geog. Journal, 1907.

[7] Harrison, W. H., and Aiyer, P. A. Subramania, “The Gases of

Swamp Rice Soils,” Mem. Dept. Agr. in India, Chem. Ser. (I.) “Their

Composition and Relationship to the Crop,” 1913, vol. iii., No.

3; (II.) “Their Utilisation for the Aeration of the Roots of the

Crop,” 1914, vol. iv., No. 1; (IV.) “The Source of the Gaseous Soil

Nitrogen,” 1916, vol. v., No. 1.

[8_a_] Hensen, V., “Ueber die Bestimmung des Planktons oder des im

Meere treibenden Materials am Pflanzen und Thieren.” Fünfter Ber.

Komm. wiss. Unters. deutschen Meere, 1887.

[8] Moore, G. T., and Karrer, J. L., “A Subterranean Alga Flora,”

Ann. Miss. Bot. Gard., 1919, vi., pp. 281-307.

[9] Nadson, G., “Die perforierenden (kalkbohrende) Algen und ihre

Bedeutung in der Natur,” Scripta bot. hort. Univ. Imp. Petrop., 1901,

Bd. 17.

[10] Petersen, J. B., “Danske Aërofile Alger,” D. Kgl. Danske

Vidensk. Selsk. Skrifter, 7 Raekke, Naturv. og mathem., 1915, Bd.

xii., 7, Copenhagen.

[11] Robbins, W. W., “Algæ in some Colorado Soils,” Agric. Exp. Sta.,

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[12] Treub, “Notice sur la nouvelle Flora de Krakatau,” Ann. Jard.

Bot. Buitenzorg, 1888, vol. vii., pp. 221-223.

II. RELATION OF ALGÆ TO LIGHT AND CARBON.

[13] Artari, A., “Zur Ernährungsphysiologie der grünen Algen,” Ber.

der D. bot. Ges., 1901, Bd. xix., S. 7.

[14] Artari, A., “Zur Physiologie der Chlamydomonaden (Chlam.

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(II.) _Ibid._, 1914, Bd. liii., S. 527.

[15] Adjarof, M., “Recherches expérimentales sur la Physiologie de

quelques Algues vertes,” Université de Genève--Institut Botanique,

Prof. R. Chodat--1905, 6 serie, vii. fascicule, Genève.

[16] Beijerinck, M. W., “Berichte über meine Kulturen niederer Algen

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xiii., S. 368, Jena.

[17] Boresch, K., “Die Färbung von Cyanophyceen und Chlorophyceen in

ihrer Abhängigkeit vom Stickstoffgehalt des Substrates,” Jahrbücher

für Wiss. Botanik., 1913, lii., pp. 145-85.

[18] Chodat, R., “Étude critique et expérimentale sur le

polymorphisme des Algues,” Genève, 1909.

[19] Chodat, R., “La crésol-tyrosinase, réactif des peptides et

des polypeptides, des protéides et de la protéolyse,” Archiv. des

Sciences physiques et naturelles, 1912.

[20] Chodat, R., “Monographie d’Algues en Culture pure: Matériaux

pour la Flore Cryptogamique Suisse,” 1913, vol. iv., fasc. 2, Berne.

[21] Dangeard, P. A., “Observations sur une Algue cultivée à

l’obscurité depuis huit ans,” Compt. Rend. Acad. Sci. (Paris), 1921,

vol. clxxii., No. 5, pp. 254-60.

[22] Étard et Bouilhac, “Sur la présence de la chlorophyll dans un

Nostoc cultivé à l’abri de la lumière,” Compt. Rend., t. cxxvii, 1898.

[23] Grintzesco, J., “Recherches expérimentales sur la morphologie et

la physiologie expérimentale de _Scenedesmus acutus_,” Meyen. Bull.

herb. Boiss., 1902, Bd. ii., pp. 219-64 and 406-29.

[24] Grintzesco, J., “Contribution à l’étude des Protococcoidées:

_Chlorella vulgaris_ Beyerinck,” Revue générale de Botanique, 1903,

xv., pp. 5-19, 67-82.

[25] * Kufferath, H., “Contribution à la physiologie d’une

protococcacée nouvelle, _Chlorella luteo-viridis_ Chod. n. sp. var.,

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1913, t. ix, p. 113.

[26] Kufferath, H., “Recherches physiologiques sur les algues vertes

cultivées en culture pure,” Bull. Soc. Roy. Bot. Belgique, 1921,

liv., pp. 49-77.

[27] Magnus, W., and Schindler, B., “Ueber den Einflusz der Nährsalze

auf die Färbung der Oscillarien,” Ber. der D. Bot. Gesellschaft,

1912-13, xxx., p. 314.

[28] * Nakano, H., “Untersuchungen über die Entwicklungs- und

Ernährungsphysiologie einiger Chlorophyceen,” Journ. College of Sci.

Imp. Univ. Tokyo, 1917, vol. xl., Art. 2.

[29] Pringsheim, E., “Kulturversuche mit chlorophyll-führenden

Mikroorganismen,” Cohns Beiträge Z. Biol. d. Pflanzen. (I.)

Die Kultur von Algen in Agar, 1912, Bd. xi., S. 249; (II.) Zur

Physiologie der _Euglena gracilis_, 1913, Bd. xii., S. 1.; (III.) Zur

Physiologie der Schizophyceen, 1913, Bd. xii., S. 99.

[30] Radais, “Sur la culture pure d’une algue verte; formation de

chlorophylle à l’obscurité,” Comptes Rendus, 1900, cxxx., p. 793.

[31] Richter, O., “Zur Physiologie der Diatomeen.” (I.) Sitzber. d.

kais. Akad. d. W. in Wien, math, naturw. Kl., 1906, Bd. cxv., Abt.

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[32] Richter, O., “Ernährung der Algen,” 1911.

[33] Robbins, W. J., “Direct Assimilation of Organic Carbon by

_Ceratodon purpureus_,” Bot. Gaz., 1918, lxv., pp. 543-51.

[34] Schindler, B., “Ueber den Farbenwechsel der Oscillarien,”

Zeitsch. f. Bot., 1913, v., pp. 497-575.

[35] Ternetz, Charlotte, “Beiträge zur Morphologie und Physiologie

der _Euglena gracilis_,” Jahrb. f. Wiss. Bot., 1912, Bd. 51, S. 435.

III. RELATION OF ALGÆ TO NITROGEN.

[36] Berthelot, “Recherches nouvelles sur les microorganismes

fixateurs de l’azote,” Comptes Rend., 1893, cxvi., pp. 842-49.

[37] Bouilhac, R., “Sur la fixation de l’azote atmosphérique par

l’association des algues et des bactéries,” Comptes Rend., 1896,

cxxiii., pp. 828-30.

[38] Bouilhac and Giustiniani, “Sur une culture de sarrasin en

présence d’un mélange d’algues et de bactéries,” Comptes Rendus,

1903, cxxxvii., pp. 1274-76.

[39] Charpentier, P. G., “Alimentation azotée d’une algue: Le

Cystococcus humicola,” Ann. Inst. Pasteur, 1903, 17, pp. 321-34.

[40] Fischer, Hugo, “Über Symbiose von Azotobacter mit Oscillarien,”

Centr. f. Bakt., 1904, xii.

[41] Frank, B., “Uber den experimentellen Nachweis der Assimilation

freien Stickstoffs durch Erdbewohnende Algen,” Ber. der D. Bot.

Gesellsch., 1889, vol. vii., pp. 34-42.

[42] Frank, B., “Ueber den gegenwärtigen Stand unserer Kenntnisse der

Assimilation elementaren Stickstoffs durch die Pflanze,” Ber. der. D.

Bot. Ges., 1889, vii., 234-47.

[43] Frank, B., and Otto, R., “Untersuchungen über Stickstoff

Assimilation in der Pflanze,” Ber. der D. Bot. Ges., 1890, viii.,

331-342.

[44] Gautier and Drouin, “Recherches sur la fixation de l’azote par

le sol et les végétaux,” Compt. Rend., 1888, cvi., pp. 1174-76;

General Conclusions, p. 1232.

[45] Kossowitsch, P., “Untersuchungen über die Frage, ob die Algen

freien Stickstoff fixiren,” Bot. Zeit., 1894, Heft 5, S. 98-116.

[46] Krüger, W., und Schneidewind, “Sind niedere chlorophyllgrüne

Algen imstande, den freien Stickstoff der Atmosphäre zu assimilieren

und Boden an Stickstoff zu bereichern?” Landwirtschaftliche Jahrb.,

1900, Bd. 29, S. 771-804.

[47] Moore, Benjamin, and T. Arthur Webster, “Studies of the

photosynthesis in f.w.a.” (I.) “The fixation of both C and N from

atmosphere to form organic tissue by green plant cell”; (II.)

“Nutrition and growth produced by high gaseous dilutions of simple

organic compounds, such as formaldehyde and methylic alcohol”; (III.)

“Nutrition and growth by means of high dilution of CO₂ and oxides of

N without access to atmosphere,” Proc. Roy. Soc., London, 1920, B.

xci., pp. 201-15.

[47_a_] Moore, B., Whiteley, Webster, T. A., Proc. Roy. Soc., London,

B., 1921; xcii., pp. 51-60.

[48] Reinke, J., “Symbiose von Volvox und Azotobacter,” Ber. der d.

Bot. Ges., 1903, Bd. xxi., S. 481.

[49] Russell, E. J., and Richards, E. H., “The washing out of

Nitrates by Drainage Water from Uncropped and Unmanured Land,” Journ.

Agric. Sci., 1920, vol. x., Part I.

[50] Schloesing, fils, and Laurent, E., “Recherches sur la fixation

de l’azote libre par les plantes,” Ann. de l’Institut Pasteur, 1892,

vi., pp. 65-115.

[51] Schramm, J. R., “The Relation of Certain Grass Green Algæ to

Elementary Nitrogen,” Ann. Mo. Bot. Gard., 1914, i., No. 2.

[52] Wann, F. B., “The Fixation of Nitrogen by Green Plants,” Amer.

Journ. Bot., 1921, viii., pp. 1-29.