CONCEPT AND ANALYSIS
=27. Definition of the habitat.= The habitat is the sum of all the
forces or factors present in a given area. It is the exact equivalent of
the term environment, though the latter is commonly used in a more
general sense. As an ecological concept, the habitat refers to an area
much more definite in character, and more sharply limited in extent than
the habitat of species as indicated in the manuals. Since the careful
study of habitats has scarcely begun, it is impossible to recognize and
delimit them in an absolute sense. Visible topographic boundaries often
exist, but in many cases, the limit, though actual, is not readily
perceived. Contiguous habitats may be sharply limited, or they may pass
into each other so gradually that no real line of demarcation can be
drawn. Whatever variations they may show, however, all habitats agree in
the possession of certain essential factors, which are universally
present. On the other hand, a few factors are merely incidental and may
be present or absent. The relative value and amount of these is probably
similar for no two habitats, though the latter readily fall into groups
with reference to the amount of some particular factor.
=28. Factors.= The factors of a habitat are water-content, humidity,
light, temperature, soil, wind, precipitation, pressure, altitude,
exposure, slope, surface (cover), and animals. To these should be added
gravity and polarity, which are practically uniform for all habitats,
and may, in consequence, be ignored in this treatise. Length of season,
while it plays an important part in vegetation, is clearly a complex and
is to be treated under its constituents. Of the factors given, all are
regularly found in each habitat, though some are not constantly present.
The first five, water-content, humidity, light, temperature, and soil
are the most important, and any one may well serve as a basis for
grouping habitats into particular classes with reference to quantity. As
will be pointed out later, however, water-content and light furnish the
most striking differences between habitats, and offer the best means of
classification. As habitats are inseparable from the formations which
they bear, the discussion of the kinds of habitats is reserved for
chapter IV.
CLASSIFICATION OF FACTORS
=29. The nature of factors.= The factors of a habitat are arranged in
two groups according to their nature: (1) physical, (2) biotic. In the
strict sense, the physical factors constitute the habitat proper, and
are the real causative forces. No habitat escapes the influence of
biotic factors, however, as the formation always reacts upon it, and the
influence of animals is usually felt in some measure. Physical factors
are further grouped into (1) climatic and (2) edaphic, with respect to
source, or, better, the medium in which they are found. Climatic, or
atmospheric factors are humidity, light, temperature, wind, pressure,
and precipitation. Axiomatically, the stimuli which they produce are
especially related to the leaf. Edaphic or soil factors are confined to
the soil, as the term denotes, and are immediately concerned with the
functions of the root. Water-content is by far the most important of
these; the others are soil composition (nutrient-content), soil
temperature, altitude, slope, exposure, and surface. The last four are
of a more general character than the others, and are usually referred to
as physiographic factors. Cover, when dead, might well be placed among
these also, but as it is little different from the living cover in
effect, it seems most logical to refer it to biotic factors.
=30. The influence of factors.= While the above classification is both
obvious and convenient, a more logical and intimate grouping may be made
upon the influence which the factor exerts. On this basis, factors are
divided into (1) direct, (2) indirect, and (3) remote. Direct factors
are those which act directly upon an important function of the plant and
produce a formative effect: for example, an increase in humidity
produces an immediate decrease in transpiration. They are water-content,
humidity, and light. Other factors have a direct action: thus
temperature has an immediate influence upon respiration and probably
assimilation also, but it is not structurally formative. Wind has a
direct mechanical effect upon woody plants, but it does not fall within
our definition. Indirect factors are those that affect a formative
function of the plant through another factor; thus a change in
temperature causes a change in humidity and this in turn calls forth a
change in transpiration; or, a change in soil texture increases the
water-content, and this affects the imbibition of the root-hairs.
Indirect factors, then, are temperature, wind, pressure, precipitation,
and soil composition. Remote factors are, for the most part,
physiographic and biotic: they require at least two other factors to act
as middlemen. Altitude affects plants through pressure, which modifies
humidity, and hence transpiration. Slope determines in large degree the
run-off during a rainstorm, thus affecting water-content and the amount
of water absorbed. Earthworms and plant parts change the texture of the
soil, and thereby the water-content. Indirect factors often exert a
remote influence also, as may be seen in the effect which temperature
and wind have in increasing evaporation from the soil, and thus reducing
the water-content. This distinction between factors may seem
insufficiently grounded. In this event, it should be noted that it
centers the effects of all factors upon the three direct ones,
water-content, humidity, and light. If it further be recalled that these
are the only factors which produce qualitative structural changes, and
that the classification of ecads and formations is based upon them, the
validity of the distinction is clear.
THE DETERMINATION OF FACTORS
=31. The need of exact measurement.= Any serious endeavor to find in the
habitat those causes which are producing modification in the plant and
in vegetation can not stop with the factors merely. The next step is to
determine the quantity of each. It is not sufficient to hazard a guess
at this, or to make a rough estimate of it. Habitats differ in all
degrees, and it is impossible to institute comparisons between them
without an exact measure of each factor. Similarly, one can not trace
the adaptations of species to their proper causes unless the quantity of
each factor is known. It is of little value to know the general effect
of a factor, unless it is known to what degree this effect is exerted.
For this purpose it becomes necessary to appeal to instruments, in order
to determine the exact amount of each factor that is present in a
particular habitat, and hence to determine the ratio between the
stimulus and the amount of structural adjustment which results. The
employment of instruments of precision is clearly indispensable for the
task which we have set for ecology, and every student that intends to
strike at the root of the subject, and to make lasting contributions to
it, must familiarize himself with instrumental methods. One great
benefit will accrue to ecology as soon as this fact is generally
recognized. The use of instruments and the application of results
obtained from them demand much patience and seriousness of purpose upon
the part of the student. As a consequence, there will be a general
exodus from ecology of those that have been attracted to it as the
latest botanical fad, and have done so much to bring it into disrepute.
=32. The value of meteorological methods.= At the outset there must be a
very clear understanding that weather records and readings have only a
very general value. This is in spite of the fact that the instruments
employed are of standard precision. An important reason for this lack of
value is that readings are not made in a particular habitat; as a rule,
indeed, they are made in towns and cities, and hence are far removed
from masses of vegetation. They are usually taken at considerable
heights, and give but a general indication of the conditions at the
level of vegetation. The chief difficulty, however, is that the factors
observed at weather stations—temperature, pressure, wind, and
precipitation—are those which have the least value for the ecologist. It
is true that a knowledge of the temperature and rainfall of a great
region will afford some idea of the general character of its vegetation.
A proper understanding of such a vegetation is, however, to be gained
only through the exact study of its component formations. Ecology has
already incurred sufficient censure as a subject composed of very
general ideas, and the use of meteorological data, which can never be
connected definitely with anything in the plant or the formation, should
be discontinued. This must not be understood to mean that meteorological
instruments can not be used in the proper place and manner, i. e., in
the habitat.
=33. Habitat determination.= It is self-evident that determinations of
factors by instruments can only be of value in the habitat where they
are made. In other words, a habitat is a unit for purposes of measuring
its factors, and measures of one habitat have no exact value in another.
This fact can not be overstated. Thus, while it is perfectly legitimate,
and indeed highly desirable, to locate thermographs in different
mountain zones for ascertaining the rate at which temperature decreases
with altitude, the data obtained in this way are not directly applicable
in explanation of plant or formation changes, except when the same
species occurs at different altitudes. Special methods are valuable and
often absolutely necessary, but in view of the fact that the plant as
well as the formation is the definite product of a definite habitat, the
fundamental rule in instrumentation is that complete readings must be
made within a habitat for that habitat alone. This necessarily
presupposes a certain preliminary acquaintance with the habitat to be
investigated, as it is imperative that the station for making readings
be located well within the formation, in order to avoid transition
conditions. In vegetation, there are as many habitats as formations, and
in addition a large number of new and denuded habitats upon which
successions have not yet started; a knowledge of each formation or
succession must rest ultimately upon the factors of its particular
habitat.
=34. Determinable and efficient differences.= The instruments employed
in studying habitats can not be too exact, as there is no adequate
knowledge as yet concerning the real differences which exist between
related or contiguous formations. This is particularly true of
differences which are efficient in producing a recognizable structural
change in plant or formation. Investigations made by the writer have
shewn that standard instruments will measure differences of quantity
quite too small to produce a visible reaction. Efficient differences are
not the same for different factors, and perhaps also for the same factor
when found in various combinations. They vary widely for different
species, being in direct relation to the plasticity of the latter. The
point necessary to bear in mind in formulating methods for habitat
investigation and in making use of instruments is that standard
instruments should be used for the very reason that we do not yet know
the relation between determinable and efficient differences. On the
other hand, it is unnecessary to insist upon absolute exactness as soon
as it is found that the determinable difference lies well within the
efficient one. This by no means indicates that instruments are not to be
carefully standardized and frequently checked, or that accurate readings
should not be made. It means that a slight margin of error may be
permitted in a machine which registers well within the efficient
difference for that factor, and that instruments that read to the last
degree of nicety are not absolutely necessary. In the fundamental work
of determining efficient differences, however, instruments can not have
too great precision. Moreover, these differences must be based upon the
most plastic species of a formation, and the readings must be made under
normal conditions.
INSTRUMENTATION
=35. Methods.= In the field use of instruments two methods have been
developed. The first in point of time was the method of simple
instruments, devised especially for class work, and capable of being
used only where a number of trained students are available. The method
of automatic instruments was an immediate outgrowth of this, due to the
necessity which confronts the solitary investigator of being in
different habitats at the same time. In the gradual evolution of this
subject, it has become possible to combine the two methods in such a way
as to retain all the advantages of the automatic method, and most of
those of the method of simple instruments.
=36. Method of simple instruments.= By simple instruments are denoted
those that do not record, but must be read by the observer at the time.
They are standard instruments of precision, but possess the disadvantage
of requiring an observer for each one. They are well illustrated by the
thermometers and psychrometers used by the Weather Bureau. In the hands
of trained observers the results obtained are unimpeachable; in fact,
standard simple instruments must be constantly employed to check
automatic ones. As physical factors vary greatly through the day and
through the year, it is all-important that the readings in habitats
which are being compared should be made at the same instant. This
requires a number of observers; as many as twelve stations have been
read at one time, and there is of course no limit to the number. It is
very important, also, that observers be carefully trained in the
handling of instruments, and in reading them accurately and
intelligently at the proper moment. In practice it has been found
impossible to do such work in elementary classes, and, even in using
small advanced classes, prolonged drill has been necessary before
trustworthy results could be obtained. When a class has once been
thoroughly trained in making accurate simultaneous readings, there is
practically no limit, other than that set by time, to the valuable work
that can be done, both in instruction and investigation.
=37. Method of automatic instruments.= The solitary investigator must
replace trained helpers by automatic instruments or ecographs. These
have the very great advantages of giving continuous simultaneous records
for long periods, and of having no personal equation. They must be
regulated and checked, to be sure, but as this is all done by the same
person, the error is negligible. There is nothing more satisfactory in
resident investigation than a series of accurate recording instruments
in various habitats. Ecographs have two disadvantages. The chief perhaps
is cost. The expense of a single “battery” which will record light,
water-content, humidity, and temperature is about $250. Another
difficulty is that they can be used only within a few miles of the base,
since they require attention every week for regulation, change of
record, etc. While this means that ecographs in their present form are
not adapted to reconnaissance, this is not a real disadvantage, as the
scattered observations possible on such a journey can best be made by
simple instruments.
=38. Combined methods.= The best results by far are to be obtained by
the combined use of simple and automatic instruments. This is
particularly true in research, but it applies also to class instruction.
The ecographs afford a continuous, accurate basal record, to which a
single reading made at any time or place can be readily referred for
comparison. On the other hand, it is an easy matter to carry a full
complement of simple instruments on the daily field trips, and to make
accurate readings in a score or more of formations in a single day. An
isolated reading, especially of a climatic factor, has little or no
value in itself, but when it can be compared with a reading made at the
same time in the base station by an ecograph, it is the equivalent of an
automatic reading. This method renders a set of simple instruments more
desirable for a long trip or reconnaissance than a battery of automatic
ones. It is practically impossible to carry the latter into the field,
and in any event a continuous record is out of the question. As there
are other tasks at such times also, it becomes evident that the taking
of single readings which can be compared with a continuous record offers
the most satisfactory solution.
CONSTRUCTION AND USE OF INSTRUMENTS
=39. The selection of instruments.= In selecting and devising
instruments for the investigation of physical factors, emphasis has
first been laid upon accuracy. This is the result of a feeling that it
is better to have instruments that read too minutely than those which do
not make distinctions that are sufficiently close, particularly until
more has been learned about efficient differences. On the other hand, no
hesitation has been felt in employing instruments which are not
absolutely accurate, when it was clear that the error was less than the
efficient difference. Similarly, the margin of error practically
eliminates itself in the case of simultaneous comparative readings, when
the instruments have been checked to the same standard. Simplicity of
construction and operation are of great importance, especially in saving
time where a large number of instruments are in operation. Expense is
likewise to be carefully considered. It is impossible to have too many
instruments, but cost practically determines the number that can be
obtained. It is further necessary to secure or invent both simple and
automatic instruments for all factors, except such invariable ones as
altitude, slope, etc. Simple instruments must be of a kind that can be
easily carried, and so constructed that they can be used at a minimum of
risk. The sling psychrometer, for example, is very readily broken in
field use, and it has been replaced by a protected modification, the
rotating form.
In describing the construction and operation of the many factor
instruments, there has been no attempt to make the treatment exhaustive.
Those instruments which the author has found of greatest value in his
own work are given precedence, and the manner of using them is described
in detail. Other instruments of value are also considered, though with
greater brevity. Some of the most complex and expensive ones have been
ignored, as it is altogether improbable that they can come into general
use in their present form. While the conviction is felt that the methods
described below will enable the most advanced investigators to carry on
thorough work, it is hoped that they will be seen to be so fundamental,
and so attractive, that they will appeal to all who are planning serious
ecological study.
WATER-CONTENT
=40. Value of different instruments.= The paramount importance of
water-content as a direct factor in the modification of plant form and
distribution gives a fundamental value to the methods used for its
determination. Automatic instruments for ascertaining the water in the
soil are costly, in addition to being complicated, and often inaccurate.
For these reasons, much attention has been given to developing the
simpler but more reliable methods in which a soil borer or geotome is
used. The latter is simple, inexpensive, and accurate. It can be carried
easily upon daily trips or upon longer reconnaissances, and is always
ready for instant use. In the determination of physiological
water-content, it is practically indispensable. Indeed, the readiness
with which geotome determinations of water-content can be made should
hasten the universal recognition of the fact that it is the available,
and not the total amount of water in the soil, which determines the
effect upon the plant.
_Geotome Methods_
[Illustration: Fig. 1. Geotomes and soil can.]
=41. The geotome.= In its simplest form, the geotome is merely a stout
iron tube with a sharp cutting edge at one end and a firmly attached
handle at the other. The length is variable and is primarily determined
by the location of the active root surface of the plant. In xerophytic
habitats, generally a longer tube is necessary than in mesophytic ones.
The bore is largely determined by the character of the soil; for
example, a larger one is necessary for gravel than for loam. Tubes of
small bore also tend to pack the soil below them, and to give a
correspondingly incomplete core. The best results have been obtained
with geotomes of ½–1 inch tube. Each geotome has a removable rod,
flattened into a disk at one end, and bent at the other, for forcing out
the core after it has been cut from the soil. Sets of geotomes have been
made in lengths of 5, 10, 12, 15, 20, and 25 inches. The 12– and 15–inch
forms have been commonly used for herbaceous formations and layers. They
are marked in inches so that a sample of any lesser depth may be readily
taken. Such a device is very necessary for gravel soils and in mountain
regions, where the subsoil of rock lies close to the surface.
[Illustration: Fig. 2. Fraenkel soil borer.]
[Illustration: Fig. 3. American soil borer.]
=42. Soil borers.= There is a large variety of soil borers to choose
from, but none have been found as simple and satisfactory for relatively
shallow readings as the geotome just described. For deep-rooted plants,
many xerophytes, shrubs, and trees, borers of the auger type are
necessary. These are large and heavy, and of necessity slow in
operation. They can not well be carried in an ordinary outfit of
instruments, and the size of the soil sample itself precludes the use of
such instruments far from the base station, except on trips made
expressly for obtaining samples from deep-seated layers. For depths from
two to eight feet, the Fraenkel borer is perhaps the most satisfactory,
except for the coarser gravels: it costs $14 or $20 according to the
length. For greater depths, or when a larger core is desirable, the
Bausch & Lomb borer, number 16536, which costs $5.25, should be made use
of. This is a ponderous affair and can be employed only on special
occasions. On account of the size of samples obtained by these borers,
it is usually most satisfactory to take a small sample from the core at
different depths. Frequently, indeed, a hand trowel may be readily used
to obtain a good sample at a particular depth.
=43. Taking samples of soil.= In obtaining soil samples, the usual
practice is to remove the air-dried surface, noting its depth, and to
sink the geotome with a slow, gentle, boring movement, in order to avoid
packing the soil. This difficulty is further obviated by deep notches
with sharp, beveled edges which are cut at the lower end. In obtaining a
fifteen-inch core, there is also less compression if it be cut five
inches at a time. Repeated tests have shown, however, that the single
compressed sample is practically as trustworthy as the one made in
sections. The water-content of the former constantly fell within .5 per
cent of that of the latter, and both varied less than 1 per cent from
the dug sample used as a check. As soon as dug, the core is pressed out
of the geotome by the plunger directly into an air-tight soil can.
Bottles may be used as containers, but tin cans are lighter and more
durable. Aluminum cans have been devised for this purpose, but on
account of the expense, “Antikamnia” cans have been used instead. These
are tested, and those that are not water-tight are rejected, although it
has been found that, even in these, ordinary soils do not lose an
appreciable amount of water in twenty-four hours. The lid should be
screwed on as quickly as possible, and, as an added precaution, the cans
are kept in a close case until they have been weighed. The cans are
numbered consecutively on both lid and side in such a way that the
number may be read at a glance. The numbers are painted, as a label
wears off too rapidly, and scratched numbers are not quickly discerned.
[Illustration: Fig. 4. Field balance.]
=44. Weighing.= Although soil samples have been kept in tight cans
outside of cases for several days without losing a milligram of
moisture, the safest plan is to make it a rule to weigh cans as quickly
as possible after bringing them in from the field. Moreover, when
delicate balances are available, it is a good practice to weigh to the
milligram. At remote bases, however, and particularly in the field, and
on reconnaissance, where delicate, expensive instruments are out of
place, coarser balances, which weigh accurately to one centigram, give
satisfactory results. The study of efficient water-content values has
already gone far enough to indicate that differences less than 1 per
cent are negligible. Indeed, the soil variation in a single square meter
is often as great as this. The greatest difference possible in the third
place, i. e., that of 9 milligrams, does not produce a difference of .1
of 1 per cent in the water-content value. In consequence, such strong
portable balances as Bausch & Lomb 12308 ($2), which can be carried
anywhere, give entirely reliable results. The best procedure is to weigh
the soil with the can. Turning the soil out upon the pan or upon paper
obviates one weighing, but there is always some slight loss, and the
chances of serious mishap are many. After weighing, the sample is dried
as rapidly as possible in a water bath or oven. At a temperature of 100°
C. this is accomplished ordinarily in twenty-four hours; the most
tenacious clays require a longer time, or a higher temperature. High
temperatures should be avoided, however, for soils that contain much
leaf mould or other organic matter, in order that this may not be
destroyed. When it is necessary on trips, soil samples can be dried in
the sun or even in the air. This usually takes several days, however,
and a test weighing is generally required before one can be certain that
the moisture is entirely gone. The weighing of the dried soil is made as
before, and the can is carefully brushed out and weighed. The weight of
aluminum cans may be determined once for all, but with painted cans it
has been the practice to weigh them each time.
=45. Computation.= The most direct method of expressing the
water-content is by per cents figured upon the moist soil as a basis.
The ideal way would be to determine the actual amount of water per unit
volume, but as this would necessitate weighing one unit volume at least
in every habitat studied, as a preliminary step, it is not practicable.
The actual process of computation is extremely simple. The weight of the
dried sample, _w_^1, is subtracted from the weight of the original
sample, _w_, and the weight of the can, _w_^2, is likewise subtracted
from _w_. The first result is then divided by the second, giving the per
cent of water, or the physical water-content. The formula is: (_w_ −
_w_^1)/(_w_ − _w_^2) = _W_. The result is expressed preferably in grams
per hundred grams of moist soil; thus ²⁰⁄₁₀₀, from which the per cent of
water-content may readily be figured on the basis of dry or moist soil.
=46. Time and location of readings.= Owing to the daily change in the
amount of soil water due to evaporation, gravity, and rainfall, an
isolated determination of water-content has very little value. It is a
primary requisite that a basis for comparison be established by making
(1) a series of readings in the same place, (2) a series at practically
the same time in a number of different places or habitats, or (3) by
combining the two methods, and following the daily changes of a series
of stations throughout an entire season, or at least for a period
sufficient to determine the approximate maximum and minimum. The last
procedure can hardly be carried out except at a base station, but here
it is practically indispensable. It has been followed both at Lincoln
and at Minnehaha, resulting in a basal series for each place that is of
the greatest importance. When such a base already exists, or, better,
while it is being established, scattered readings may be used somewhat
profitably. As a practical working rule, however, it is most convenient
and satisfactory to make all determinations consecutively, i. e., in a
series of stations or of successive days. Under ordinary conditions, the
time of day at which a particular sample is taken is of little
importance, as the variation during a day is usually slight. This does
not hold for exposed wet soils, and especially for soils which have just
been wetted by rains. In all comparative series, however, the samples
should be taken at the same hour whenever possible. This is particularly
necessary when it is desired to ascertain the daily decrease of
water-content in the same spot. In the case of a series of stations,
these should be read always in the same order, at the same time of day,
and as rapidly as possible. When a daily station series is being run, i.
e., a series by days and stations both, the daily reading for each place
should fall at the same time. While there are certain advantages in
making readings either early or late in the day, they may be made at any
time if the above precautions are followed.
=47. Location of readings.= Samples should invariably be taken in spots
which are both typical and normal, especially when they are to be used
as representative of a particular area or habitat. A slight change in
slope, soil composition, in the amount of dead or living cover, etc.,
will produce considerable change in the amount of water present. Where
habitat and formation are uniform, fewer precautions are necessary. This
is a rare circumstance, and as a rule determinations must be made
wherever appreciable differences are in evidence. The problem is simpler
when readings are taken with reference to the structure or modifications
of a particular species, but even here, check readings in several places
are of great value. The variation of water in a spot apparently uniform
has been found to be slight in the prairies and the mountains. In taking
three samples in spots a few inches to several feet apart, the
difference in the amount of water has rarely exceeded 1 per cent, which
is practically negligible. Gardner[2] found that 16 samples taken to a
depth of 3 inches, in as many different portions of a carefully
prepared, denuded soil plot, showed a variation of 7½ per cent. This is
partially explained by the shallowness of the samples, but even then the
results of the two investigations are in serious conflict and indicate
that the question needs especial study. It should be further pointed out
that all readings should be made well within a particular area, and not
near its edge, and that, in the case of large diversified habitats, it
is the consocies and the society which indicate the obvious variations
in the structure of the habitat.
=48. Depth of samples.= The general rule is that the depth of soil
samples is determined by the layer to which the roots penetrate. The
practice is to remove the air-dried surface in which no roots are found,
and to take a sample to the proper depth. This method is open to some
objection, as the actively absorbing root surfaces are often localized.
There is no practical way of taking account of this as yet, except in
the case of deep-rooted xerophytes and woody plants. It is practicable
to determine the location of the active root area of a particular plant
and hence the water-content of the soil layer, but in most formations,
roots penetrate to such different depths that a sample which includes
the greater part of the distance concerned is satisfactory. Some
knowledge of the soil of a formation is also necessary, since shallow
soils do not require as deep samples as others. The same is true of
shaded soils without reference to their depth, and, in large measure, of
soils supplied with telluric water. In all cases, it is highly desirable
to have numerous control-samples at different depths. The normal cores
are 12 or 15 inches; control-samples are taken every 5 inches to the
depth desired, and in some cases 3–inch sections are made. It has been
found a great saving of time to combine these methods. A 5–inch sample
is taken and placed in one can, then a second one, and a third in like
manner. In this way the water-content of each 5–inch layer is
determined, and from the combined weight the total content is readily
ascertained.
=49. Check and control instruments.= A number of instruments throw much
light upon the general relations of soil water. The rain-gauge, or
ombrometer, measures the periodical replenishment of the water supply,
and has a direct bearing upon seasonal variation. The atmometer affords
a clue to the daily decrease of water by evaporation, and thus
supplements the rain-gauge. The run-off gauge enables one to establish a
direct connection between water-content and the slope and character of
the surface. The amount and rapidity of absorption are determined by
means of a simple instrument termed a rhoptometer. The gravitation water
of a soil is ascertained by a hizometer, and some clue to the
hygroscopic and capillary water may be obtained by an artificial osmotic
cell. All of these are of importance because they serve to explain the
water-content of a particular soil with especial reference to the other
factors of the habitat. It is evident that none of them can actually be
used in exact determinations of the amount of water, and they will be
considered under the factors with which, they are more immediately
concerned.
_Physical and Physiological Water_
=50. The availability of soil water.= The amount of water present in a
soil is no real index to the influence of water-content as a factor of
the habitat. All soils contain more water than can be absorbed by the
plants which grow in them. This residual water, which is not available
for use, varies for different soils. It is greatest in the compact
soils, such as clay and loam, and least in the loose ones, as sand and
gravel. It differs, but to a much less degree, from one species to
another. A plant of xerophytic tendency is naturally able to remove more
water from the same soil than one of mesophytic or hydrophytic
character. As the species of a particular formation owe their
association chiefly to their common relation to the water-content of the
habitat, this difference is of little importance in the field. In
comparing the structure of formations, and especially that of the plants
which are found in them, the need to distinguish the available water
from the total amount is imperative. Thus, water-contents of 15 per cent
in gravel and in clay are in no wise comparable. A coarse gravel
containing 15 per cent of water is practically saturated. The plants
which grow upon it are mesophytes of a strong hydrophytic tendency, and
they are able to use 14½ or all but .5 out of the 15 per cent of water.
In a compact clay, only 3½ of the 15 per cent are available, and the
plants growing in it are marked xerophytes. It is evident that a
knowledge merely of the physical water-content is actually misleading in
such cases, and this holds true of comparisons of any soils which differ
considerably in texture. After one has determined the physiological
water for the great groups of soils, it is more or less possible to
estimate the amounts in the various types of each. As an analysis is
necessary to show how close soils are in texture, this is little better
than a guess, and for accurate work it is indispensable that the
available water be determined for each habitat. Within the same
formation, however, after this has once been carefully ascertained, it
is perfectly satisfactory to convert physical water-content into
available by subtracting the non-available water, which under normal
conditions in the field remains practically the same.
The importance of knowing the available water is even greater in those
habitats in which salts, acids, cold, or other factors than the
molecular attraction of soil particles increase the amount of water
which the plant can not absorb. Few careful investigations of such soils
have yet been made, and the relation of available to non-available water
in them is almost entirely unknown. It is probable that the phenomena in
some of these will be found to be produced by other factors.
=51. Terms.= The terms, physiological water-content, and physical
water-content, are awkward and not altogether clear in their
application. It is here proposed to replace them by short words which
will refer directly to the availability of the soil water for absorption
by the plant. Accordingly, the total amount of water in the soil is
divided into the available and the non-available water-content. The
terms suggested for these are respectively, _holard_ (ὅλos, whole,
ἅpδov, water), _chresard_ (χοῆςις, use), and _echard_ (ἕχω, to
withhold).
=52. Chresard determinations under control.= The determination of the
chresard in the field is attended with peculiar difficulties. In
consequence, the method of obtaining it under control will first be
described. The inquiry may be made with reference to soils in general or
to the soil of a particular formation. In the last case, if the plants
used are from the same formation, the results will have almost the value
of a field determination. When no definite habitat is the subject of
investigation, an actual soil, and not an artificial mixture, should be
used, and the plants employed should be mesophytes. The individual
plants are grown from seeds in the proper soils, and are repotted
sufficiently often to keep the roots away from the surface. The last
transfer is made to a pot large enough to permit the plant to become
full-grown without crowding the roots. The pot should be glazed inside
and out in order to prevent the escape of moisture. This interferes
slightly with the aeration of the soil, but it will not cause any real
difficulty. The plant is watered in such a way as to make the growth as
normal as possible. After it has become well established, three soil
samples are taken in such a manner that they will give the variation in
different parts of the pot. One is taken near the plant, the second
midway between the plant and the edge of the pot, and the third near the
edge. The depth is determined by the size of the pot and the position of
the roots. The holard is determined for these in the usual way, but the
result is expressed with reference to 100 grams of dry soil; the average
is taken as representative. The soil is then allowed to dry out slowly,
as sudden drouth will sometimes impair the power of absorption and a
plant will wilt although considerable available water remains. Plants
often wilt in the field daily for several successive hot dry days, and
become completely turgid again during the night. If the drying out takes
place slowly, the plant will not recover after it has once begun to
wilt. The proper time to make the second reading is indicated by the
pronounced wilting of the leaves and shoots. Complete wilting occurs, as
a rule, only after the younger parts have drawn for some time upon the
watery tissues of the stem and root, by which time evaporation has
considerably deceased the water in the soil. It is a well-known fact
that young leaves do not wilt easily, especially in watery or succulent
plants. Three samples are again taken and the average water-content
determined as above. This is the non-available water or the echard. The
latter is then computed on the basis of 100 grams of dry soil, and this
result is subtracted from the holard to give the chresard in grams for
each 100 grams of dry weight. The chresard may also be expressed with
respect to 100 grams of moist soil. As a final precaution in basal work,
it is advisable to determine the chresard for six individuals of the
same species under as nearly the same conditions as possible. When it is
desired, however, to find the average chresard for a particular soil, it
is necessary to employ various species representing diverse phyads and
ecads. Such an investigation is necessarily very complicated, and must
be made the subject of special inquiry.
=53. Chresard readings in the field.= The especial difficulties which
must be overcome in the field are the exclusion of rain and dew and the
cutting off of the capillary water. It is evident, of course, that
experiments of this sort must also be entirely free from outside
disturbance. The choice of an area depends upon the scope of the study.
If the chresard is sought for a particular consocies, the block of soil
to be studied should show several species which are fairly
representative. In case the chresard of a certain species is to be
obtained, this species alone need be present, but it should be
represented by several individuals. Check plots are desirable in either
event, and at least two or three which are as nearly uniform as possible
should be chosen. The size and depth of the soil block depends upon the
plants concerned. It must be large enough that the roots of the
particular individuals under investigation are not disturbed. There is a
limit to the size of the mass that can be handled readily, and in
consequence the test plants must not be too large or too deeply rooted.
The task of cutting out the soil block requires a spade with a long
sharp blade. After ascertaining the spread and depth of the roots, the
block is cut so that a margin of several inches free from the roots
concerned is left on the sides and bottom. If the block is to be lifted
out of place, so that the sides are exposed to evaporation, this
allowance should be greater. In some cases, it may be found more
convenient to dig the plant up, place it in a large pot, and put the
latter back in the hole. As a general practice, however, this is much
less satisfactory.
After the block has been cut, it may be moved if the soil is
sufficiently compact, and then allowed to dry out in its own formation
or elsewhere. The results are most valuable in the first case, though it
is often an advantage to remove blocks cut from shade or wet formations
to dry, sunny stations where they will dry more rapidly. The most
satisfactory and natural method, however, is to leave the block in
place, and to prevent the reestablishment of capillary action by
enclosing it within plates. This is accomplished by slipping thin
sheet-iron plates into position along the cut surfaces. The plate for
the bottom should be somewhat wider than the block, and is slipped into
place by raising the block if the soil is not too loose; in the latter
event, it is carefully driven in. The side plates are then pushed down
to meet the former. The size of the plates depends upon the block; in
general, plates of 1, 2, and 3 feet square, with the bottom plates a
trifle larger, are the most serviceable. Access of rain and dew is
prevented by an awning of heavy canvas which projects far enough beyond
each side of the block to prevent wetting. The height will depend of
course upon the size of the plants. The awning must be used only when
rain or heavy dew is threatened, as the shade which it produces changes
the power of the plant to draw water from the soil.
The time necessary to cause wilting varies with the habitat and the
weather. When the block is large and in position, two or three weeks are
required. This period of drying incidentally furnishes an excellent
opportunity for determining the rate at which the particular soil loses
water. The holard sample is taken daily for several days before the
block is cut out, in order to obtain an average, care being taken of
course to avoid a period of extreme weather. The echard samples are
taken as soon as the wilting is sufficient to indicate that the limit of
available water is reached. The air-dry soil above the roots is first
removed. The treatment of the samples and the computation of the
chresard are as previously indicated.
=54. Chresard values of different soils.= The following table gives the
water-content values of six representative soils. The per cents of
holard (at saturation) and of echard are those determined by Hedgcock[3]
with six mesophytes as test plants for each soil. The chresard has been
computed directly from these.
══════╤═════════╤═════════╤═════════
│ HOLARD │ ECHARD │CHRESARD
──────┼────┬────┼────┬────┼────┬────
Sand │14.3│12.6│ .3│ .25│ 14│12.3
Clay │47.4│32.5│ 9.3│ 6.3│38.1│26.2
Loess │59.3│37.1│10.1│ 6.4│49.2│30.7
Loam │64.1│39.1│10.9│ 6.6│53.2│32.5
Humus │65.3│39.6│11.9│ 7.2│53.4│32.4
Saline│68.5│40.8│16.2│ 9.6│52.3│31.2
──────┴────┴────┴────┴────┴────┴────
The first column indicates the per cent based upon the dry weight, the
second upon the weight of the moist soil.
While these can not be considered absolute for a particular soil other
than the ones investigated, they are found to correspond somewhat
closely to the results obtained for other soils of the respective
groups. For accurate research, the chresard must of course be
ascertained for each formation with respect to its peculiar plants and
soil. The influence of the ecad in more or less determining the echard
is also shown by Hedgcock, who found that floating plants wilt at 25 per
cent, amphibious ones at 15–20 per cent, mesophytes at 6–12 per cent,
and mesophytic xerophytes at 3–6 per cent. The echard is also somewhat
higher for shade plants than for heliophytes.
_Records and Results_
=55. The field record.= It is superfluous to point out that a definite
form for field records saves much time and prevents many mistakes. The
exact form may be left to personal taste, but there are certain features
which are essential. Many of these are evident, while others may seem
unnecessary; all, however, have been proved by experience to have some
value in saving time or in preventing confusion. The two fundamental
maxims of field work are that nothing is too trivial to be of
importance, and that no detail should be entrusted to the memory. The
field record should contain in unmistakable terms all that the field has
yielded. These statements apply with especial force to water-content, in
many senses the most important of physical factors. The precise
character of the record depends upon the way in which the readings are
made, whether scattered or in series. As the day-station series is of
the greatest importance, the record is adapted for it especially, but it
will also serve for all readings. The record is chronological, since
this is the only convenient method for the field. A proper form for a
field record of water-content is the following:
═══╤══════╤═════════╤════════╤═══════════╤════════╤══════
Can│ Date │Formation│Station │ Community │ Soil │Sample
No.│ │ │ │ │ │
───┼──────┼─────────┼────────┼───────────┼────────┼──────
„ │ „ │ „ │ „ │ „ │ „ │ „
───┼──────┼─────────┼────────┼───────────┼────────┼──────
„ │ „ │ „ │ „ │ „ │ „ │ „
───┼──────┼─────────┼────────┼───────────┼────────┼──────
10 │2/8/04│Spruce │Jack │Mertensiare│ Loam│10
│ │forest │Brook │ │ │
17 │ „ │Spruce │Milky │Gentianare │ ¼ mold│10:2
│ │forest │Way │ │¾ gravel│
40 │ „ │Gravel │Hiawatha│Asterare │ Gravel│2:10
│ │slide. │ │ │ │
───┴──────┴─────────┴────────┴───────────┴────────┴──────
═══╤═══════════════════╤════════════════════╤════════╤═══════════════
Can│ HOLARD │ ECHARD │Chresard│ NOTES
No.│ │ │ │
───┼────────────────┬──┼─────────────────┬──┼────────┼──────┬────────
„ │ Weighings │% │ Weighings │% │ „ │ Sky │Rainfall
───┼─────┬────┬─────┼──┼─────┬─────┬─────┼──┼────────┼──────┼────────
„ │ 1st │ 2d │ Can │„ │ 1st │ 2d │ Can │„ │ „ │ „ │ „
───┼─────┼────┼─────┼──┼─────┼─────┼─────┼──┼────────┼──────┼────────
10 │58.7 │50.1│25.52│ 2│58.7 │53.41│25.52│10│ 16│Cloudy│ 0
│ │ │ │ │ │ │ │ │ │ │
17 │64.25│57.5│21.35│16│64.25│59.6 │21.35│ 5│ 11│Cloudy│ 0
│ │ │ │ │ │ │ │ │ │ │
40 │78.55│74.3│22.85│ 8│78.55│74.85│22.85│ 1│ 7│Cloudy│ 0
│ │ │ │ │ │ │ │ │ │ │
───┴─────┴────┴─────┴──┴─────┴─────┴─────┴──┴────────┴──────┴────────
A general designation of the soil composition is a material aid,
especially where there is a difference in the core. For example, in a
mountain forest or meadow, the upper layer will usually be mold, the
lower sand or gravel. A careful estimate of the relation between the two
throws much light upon the chresard. Under “sample” the number taken to
reach the desired depth, if more than one, is indicated by placing the
number before the depth, thus 2:10. When two or more full cores are
included in the same sample for a check, the order is 10:2. It has
already been shown, however, that these precautions are not necessary
for ordinary purposes. In computing the holard and echard, there is no
need to show the figuring, if the process is checked and then proved.
Notes upon sky conditions aid in explaining the daily decrease in
water-content. The amount of rain and the period during which it falls
are of great importance in understanding the fluctuations of the holard.
Under community it is highly desirable to have a list of all the
species, but it is impossible to include this in the table, and a glance
at the formation list will show them. The form indicated above serves
for a day-station series, a daily series in one station for any number
of check series in one spot, and for scattered readings. In many cases
the echard will not be determined, but on account of its primary
importance, there should be a space for it, especially since it may be
desirable to determine it at some later time.
=56. The permanent record.= This should be kept by formation, or if the
latter exhibits well-defined associations, the formational record may be
divided accordingly. This may seem an unnecessary expenditure of time,
but a slight experience in finding the water-content values of a
particular habitat, when scattered through a chronological field record,
will be convincing. The form of permanent record is the same as for the
field, except that the column for the formation and that for the society
are often unnecessary.
=57. Sums and means.= From the great difficulty of determining the
absolute water-content, and of obtaining a standard of comparison
between soils on account of the varying ratio between bulk and weight,
water-content sums are impracticable. For the same reasons, means of
actual water-content are practically impossible, and the mean
water-content must be expressed in per cents. Daily readings are not
essential to a satisfactory mean. In fact, a single reading at each
extreme enables one to approximate the real mean very closely; thus, the
average of 26 readings in the prairie formation is 18 per cent. The
extremes are 5 per cent and 28 per cent, and their average 16.5 per
cent. A few readings properly scattered through moist and dry periods
will give a reliable mean, as will also a series of daily readings from
one heavy rain through a long dry period. The one difficulty with the
last method is that such periods can not well be determined beforehand.
Means permit ready comparison between habitats, but in connecting the
modifications of a species with water-content as a cause, the extremes
are significant as indicating the range of conditions. Furthermore, the
extremes, i. e., 5 per cent and 28 per cent, make it possible to
approximate the mean, 18 per cent, while the latter gives little or no
clue to the extremes. It is hardly necessary to state that means and
extremes should be determined for a certain habitat, or particular area
of it, and that the results may be expressed with reference to holard
and chresard.
=58. Curves.= The value of graphic methods and the details of plotting
curves are reserved for a particular section. It will suffice in this
place to indicate the water-content curves that are of especial value.
Simple curves are made with regard to time, place, or depth. The day
curve shows the fluctuations of the water-content of one station from
day to day or from time to time. The station curve indicates the
variation in water from station to station, while the depth curve
represents the different values at various depths in the same station.
These may be combined on the same sheet in such a way that the station
curves of each day may be compared directly. Similar combinations may be
used for comparing the day curves, or the depth curves of different
stations, but these are of less importance. A combination of curves
which is of the greatest value is one which admits of direct comparison
between the station curves of saturation, holard, chresard, and echard.
HUMIDITY
=59. Instruments.= As a direct factor, humidity is intimately connected
with water-content in determining the structure and distribution of
plants. The one is in control of water loss; the other regulates water
supply. Humidity as a climatic factor undergoes greater fluctuation in
the same habitat, and the efficient difference is correspondingly
greater. Accordingly, simple instruments are less valuable than
automatic ones, since a continuous record is essential to a proper
understanding of the real influence of humidity. As is the rule,
however, the use of simple instruments, when they can be referred to an
ecographic basis, greatly extends the field which can be studied. In
investigation, both psychrometer and psychrograph have their proper
place. In the consideration of simple instruments for obtaining humidity
values, an arbitrary distinction is made between psychrometers and
hygrometers. The former consist of a wet and a dry bulb thermometer,
while the latter make use of a hygroscopic awn, hair, or other object.
_Psychrometers_
=60. Kinds.= There are three kinds of psychrometer, the sling, the cog,
and the stationary. All consist of a wet bulb and a dry bulb thermometer
set in a case; the first two are designed to be moved or whirled in the
air. The same principle is applied in each, viz., that evaporation
produces a decrease in temperature proportional to the amount of
moisture in the air. The dry bulb thermometer is an ordinary
thermometer, while the wet bulb is covered with a cloth that can be
moistened. The former indicates the normal temperature of the air, the
latter gives the reduced temperature due to evaporation. The relative
humidity of the air is ascertained by means of the proper tables, from
two terms, i. e., the air temperature and the amount of reduction shown
by the wet bulb. The sling and the cog psychrometers alone are in
general use. The stationary form has been found to be unreliable,
because the moisture, as it evaporates from the wet bulb, is not
removed, and, in consequence, hinders evaporation to the proper degree.
[Illustration: Fig. 5. Sling psychrometer.]
=61. The sling psychrometer.= The standard form of this is shown in the
illustration, and is the one used by the Weather Bureau. This instrument
can be obtained from H. J. Green, 1191 Bedford Ave., Brooklyn, or Julien
P. Friez, 107 E. German St., Baltimore, at a cost of $5. It consists of
a metal frame to which are firmly attached two accurately standardized
thermometers, reading usually from –30° to 130°. The frame is attached
at the uppermost end to a handle in such fashion that it swings freely.
The wet bulb thermometer is placed lower, chiefly to aid in wetting the
cloth more readily. The cloth for the wet bulb should be always of the
same texture and quality; the standard used by the Weather Bureau can be
obtained from the instrument makers. A slight difference in texture
makes no appreciable error, but the results obtained with different
instruments and by different observers will be more trustworthy and
comparable if the same cloth be used in all cases. The jacket for the
wet bulb may be sewed in the form of a close-fitting bag, which soon
shrinks and clings tightly. It may be made in the field by wrapping the
cloth so that the edges just overlap, and tying it tightly above and
below the bulb. In either case, a single layer of cloth alone must be
used. The cloth becomes soiled or thin after a few months’ constant use
and should be replaced. It is a wise precaution to carry a small piece
of psychrometer cloth in the field outfit.
[Illustration: Fig. 6. Cog psychrometer.]
=62. Readings.= All observations should be made facing the wind, and the
observer should move one or two steps during the reading to prevent the
possibility of error. The cloth of the wet bulb is moistened with water
by means of a brush, or, much better, it is dipped directly into a
bottle of water. Distilled water is preferable, as it contains no
dissolved material to accumulate in the cloth. Tap-water and the water
of streams may be used without appreciable error, if the cloth is
changed somewhat more frequently. The temperature of the water is
practically negligible under ordinary conditions. Readings can be made
more quickly, however, when the temperature is not too far from that of
the air. The psychrometer is held firmly and swung rapidly through the
air when the space is not too confined. Where there is danger of
breakage, it is swung back and forth through a short arc,
pendulum-fashion. As the reading must be made when the mercury of the
wet bulb reaches the lowest point, the instrument is stopped from time
to time and the position of the column noted. The lowest point is often
indicated by the tendency of the mercury to remain stationary; as a rule
it can be noted with certainty when the next glance shows a rise in the
column. In following the movement, and especially in noting the final
reading, great care must be taken to make the latter before the mercury
begins to rise. For this reason it is desirable to shade the
psychrometer with the body when looking at it, and to take pains not to
breathe upon the bulbs nor to bring them too near the body. At the
moment when the wet bulb registers the lowest point, the dry bulb should
be read and the results recorded.
=63. Cog psychrometer.= This instrument, commonly called the
“egg-beater” psychrometer, has been devised to obviate certain
disadvantages of the sling psychrometer in field work, and has entirely
supplanted the latter in the writer’s own studies. It is smaller, more
compact, and the danger of breaking in carriage or in use is almost nil.
It has the great advantage of making it possible to take readings in a
layer of air less than two inches in thickness, and in any position.
Fairly accurate results can even be obtained from transpiring leaves.
The instrument can readily be made by a good mechanic, at a cost for
materials of $1.75, which is less than half the price for the sling
form. A single drawback exists in the use of short, Centigrade
thermometers, inasmuch as tables of relative humidity are usually
expressed in Fahrenheit. It is a simple matter, however, to convert
Centigrade degrees into Fahrenheit, mentally, or the difficulty may be
avoided by the conversion table shown on page 47, or by constructing a
Centigrade series of humidity tables. The fact that the wet and dry
bulbs revolve in the same path has raised a doubt concerning the
accuracy of the results obtained with this instrument. Repeated
comparisons with the sling psychrometer have not only removed this doubt
completely, but have also proved that the standardization of the
thermometers has been efficient.
=64. Construction and use.= A convenient form of egg-beater is the Lyon
(Albany, New York), in which the revolving plates can be readily
removed, leaving the axis and the frame. The thermometers used are of
the short Centigrade type. They are 4½ inches long and read from –5° to
50°. Eimer and Amend, 205 Third Ave., New York city, furnish them at 75
cents each. The thermometers are carefully standardized and compared,
and then grouped in pairs that read together. Each pair is used to
construct a particular psychrometer. Each thermometer is strongly wired
to one side of the frame, pieces of felt being used to protect the tube
and increase the contact. The frame is also bent at the base angles to
permit free circulation of air about the thermometer bulbs. The bulb of
one thermometer is covered with the proper cloth, and the psychrometer
is finished. Since the frame revolves with the thermometers, it is
necessary to pour the water on the wet bulb, or to employ a pipette or
brush. The thermometer bulbs are placed in the layer to be studied, and
the frame rotated at an even rate and with moderate rapidity. The
observation is further made as in the case of the sling psychrometer. As
the circle of rotation is less than three inches in diameter, and the
layer less than an inch, in place of nearly three feet for the sling
form, the instrument should not be moved at all for extremely localized
readings, but it must be moved considerably, a foot or more, if it is
desirable to obtain a more general reading.
=65. Hygrometers.= While there are instruments designed to indicate the
humidity by means of a hygroscopic substance, not one of them seems to
be of sufficient accuracy for use in ecological study. The difficulty is
that the hygroscopic reaction is inconstant, rather than that the
instruments are not sufficiently sensitive. A number of hygrometers have
been tested, and in all the error has been found to be great, varying
usually from 10–20 per cent. In the middle of the scale they sometimes
read more accurately, but toward either extreme they are very inexact.
It seems probable that an accurate hygrometer can be constructed only
after the model of the Draper psychrograph. Its weight and bulk would
make it an impossible instrument for field trips, and the expense of one
would provide a dozen psychrometers. In consequence, it does not seem
too sweeping to say that no hygrometer can furnish trustworthy results.
Of simple instruments for humidity, the psychrometer alone can be
trusted to give reliable readings. Crova’s hygrometer, used by
Hesselmann, is not a hygrometer in the sense indicated. As it is much
less convenient to handle and to operate than the cog psychrometer, it
is not necessary to describe it.
_Psychrographs_
[Illustration: Fig. 7. Draper psychrograph.]
=66. The Draper psychrograph.= A year’s trial of the Draper psychrograph
in field and planthouse has left little question of its accuracy and its
great usefulness. Essentially, it consists of a band of fine catgut
strings, which are sensitive to changes in the moisture-content of the
air. The variations in the length of the band are communicated to a long
pointer carrying an inking pen. The latter traces the record in per cent
of relative humidity on a graduated paper disk, which is practically the
face of an eight-day clock. The whole is enclosed in a metal case with a
glass front. A glance at the illustration will show the general
structure of the instrument. Continued psychrometric tests demonstrate
that the margin of error is well within the efficient difference for
humidity, which is taken to be 5 per cent. In the field tests of the
past summer, two psychrographs placed side by side in the same habitat
did not vary 1 per cent from each other. The same instruments when in
different habitats did not deviate more than 1 per cent from the
psychrometric values, except when the air approached saturation. For
humidities above 90 per cent, the deviation is considerable, but as
these are temporary and incident upon rainfall, the error is not
serious. For humidities varying from 10–85 per cent, the psychrograph is
practically as accurate as the psychrometer. Per cents below 10 are
rare, and no tests have been made for them.
[Illustration: Fig. 8. Instrument shelter, showing thermograph and
psychrograph in position.]
=67. Placing the instrument.= The psychrograph should be located in a
place where the circulation of the air is typical of the station
observed. A satisfactory shelter will screen the instrument from sun and
rain, and at the same time permit the air to pass freely through the
perforations of the metal case. The form shown in figure 8 meets both of
these conditions. A desirable modification is effected by fastening a
strip about the cover of such depth as to prevent the sun’s rays from
striking the case except when the sun is near the horizon. A cross block
is fastened on the post of the shelter after being exactly leveled. The
psychrograph rests upon this block, which is three feet above the ground
in order to avoid the influence of radiation. The instrument is held in
position by slipping the eye over a small-headed nail driven obliquely.
It does not hang from the latter, but must rest firmly upon the cross
block. The post is set to a depth that prevents oscillation in the wind,
which is liable to obscure the record. In shallow mountain soils
stability is attained by fastening a broad board at the base of the post
before setting it. When two or more psychrographs are established in
different habitats, great pains are taken to set them up in exactly the
same way. The shelters are alike, the height above the soil the same,
and the instruments all face the south.
=68. Regulating and operating the instrument.= When two or more
psychrographs are to be used in series, they must be compared with each
other in the same spot for several days until they run exactly together
with respect to per cent of humidity and to time. During this comparison
they are checked by the psychrometer and so regulated that they register
the proper humidity. When a single instrument is used alone as the basis
to which simple readings may be referred, all regulating may well be
done after the instrument is in position. This is a simple process; it
is accomplished by obtaining the relative humidity beneath the shelter
and at the proper height by a psychrometer. The pen hand is then moved
to the proper line on the disk by means of the screws at its base. These
are reached by removing the lettered glass face. The thumbscrew on the
side opposite the direction in which the pen is to move is released, and
the opposite screw simultaneously tightened, until the pen remains upon
the proper line. Experience has proved that the record sheet should be
correctly labeled and dated before being placed on the disk. In the
press of field duties, records labeled after removal are liable to be
confused. It is likewise a great saving of time to write the date of the
month in the margin of each segment. Care is taken to place the sheet on
the disk in the same position each time; this can easily be done by
seeing that the sharp point on the disk penetrates the same spot on the
paper. A single drop of ink in the pen will usually give the most
satisfactory line. A thin line is read most accurately. If the pen point
is too fine, however, the ink does not flow readily, and the point
should be slightly blunted by means of a file. More often the line is
too broad and the pen must be carefully pointed. Occasionally the pen
does not touch the sheet, and it becomes necessary to bend the hand
slightly. This is a frequent difficulty if the records are folded or
wrinkled, and consequently the sheets should always be kept flat.
=69. The weekly visit.= Psychrographs must be visited, checked, rewound,
and inked every week. Whenever possible this should be done regularly at
a specified day and hour. This is especially desirable if the same
record sheet is used for more than one week. Time and energy are saved
by a fixed order for the various tasks to be done at each visit. After
opening the instrument the disk is removed, and the clock wound, and, if
need be, regulated. The record sheet is replaced, the disk again put on
the clock arbor, and the pen replenished with a drop of ink. A
psychrometer reading is made, and the results in terms of relative
humidity noted at the proper place on the disk sheet. If the
psychrograph vary more than 1 per cent, it is adjusted to read
accurately. In practice it has been found a great convenience to keep
each record sheet in position for three weeks, and the time may easily
be extended to four. In this event, the pen is carefully cleaned with
blotting paper at each visit, and is then refilled with an ink of
different color. To prevent confusion, the three different colored inks
are always used in the same order, red for the first week, blue for the
second, and green for the third. The advantages of this plan are
obvious: fewer records are used and less time is spent in changing them.
The records of several weeks are side by side instead of on separate
sheets, and in working over the season’s results, it is necessary to
handle but a third as many sheets.
The Draper psychrograph is made by the Draper Manufacturing Company, 152
Front St., New York city. The price is $30. A few record sheets and a
bottle of red ink are furnished with it. Additional records can be
obtained at 3 cents each. The inks are 25–50 cents per bottle, depending
upon the color.
_Humidity Readings and Records_
=70. The time of readings.= If simple instruments alone are used for
determining humidity, readings are practically without value unless made
simultaneously through several stations, or successively at one. When it
is possible to combine these, and to make psychrometer readings at
different habitats for each hour of the day, or at the same hour for
several days, the series is of very great value. Single readings are
unreliable on account of the hourly and daily variations of humidity,
but when these changes are recorded by a psychrograph, such readings at
once become of use, whether made in the same habitat with the recording
instrument or elsewhere. In the latter case, one reading will tell
little about the normal humidity of the habitat, but several make a
close estimate possible. When a series of psychrographs is in use,
accurate observations can be made to advantage anywhere at any time. As
a rule, however, it has been found most convenient to make simple
readings at 6:00 A.M., 1:00 P.M., and 6:00 P.M., as these hours afford
much evidence in regard to the daily range. A good time also is that at
which the temperature maximum occurs each day, but this is movable and
in the press of field work can rarely be taken advantage of. A very fair
idea of the daily mean humidity is obtainable by averaging the readings
made at the hours already indicated. The comparison of single readings
with the psychrograph record should not be made at a time when a rapid
change is occurring, as the automatic instrument does not respond
immediately. Such a condition is usually represented by a sudden rain,
and is naturally not a satisfactory time for single readings in any
event.
[Illustration: Fig. 9. Atmometer.]
=71. Place and height.= As stated above, the psychrograph is placed
three feet above the surface of the ground in making readings for the
comparison of stations. In low, herbaceous formations, the instrument is
usually placed within a few inches of the soil in order to record the
humidity of the air in which the plants are growing. In forest
formations, the moisture often varies considerably in the different
layers. This variation is easily determined by simultaneous psychrometer
readings in the several layers, or, if occasion warrants, a series of
psychrographs may be used. In field work the rule has been to make
observations with the psychrometer at 6 feet, 3 feet, and the surface of
the soil, but the reading at the height of 3 feet is ordinarily
sufficient. Humidity varies so easily that several readings in different
parts of one formation are often desirable. In comparing different
formations, the readings should be made in corresponding situations, for
example, in the densest portion of each.
=72. Check instruments.= Humidity is so readily affected by temperature,
wind, and pressure, that a knowledge of these factors is essential to an
understanding of its fluctuations. Pressure, disregarding daily
variation, is taken account of in the tables for ascertaining relative
humidity, and is determined once for all when the altitude of a station
has been carefully established. The temperature is obtained directly
from the dry bulb reading. Its value is fundamental, as the amount of
moisture in a given space is directly affected by it; like pressure, it
also is taken account of in the formula. The movement of the air has an
immediate influence upon moisture by mixing the air of different
habitats and layers. So far as the plant is concerned, it has
practically the effect of increasing or decreasing the humidity by the
removal of the air above it. Thus, while the anemometer can furnish no
direct evidence as to the amount of variation, it is of aid in
explaining the reason for it. Likewise, the rate of evaporation as
indicated by a series of atmometers, affords a ready method of
estimating the comparative effect of humidity in different habitats.
Potometers and other instruments for measuring transpiration throw much
light upon humidity values. Since they are concerned with the response
of the plant to humidity, they are considered in the following chapter.
=73. Humidity tables.= To ascertain the relative humidity, the
difference between the wet and dry bulb readings is obtained. This, with
the dry bulb temperature, is referred to the tables, where the
corresponding humidity is found. A variation in temperature has less
effect than a variation in the difference; in consequence, the dry bulb
reading is expressed in the nearest unit, and the difference reckoned to
the nearest .5. The humidity varies with the air pressure. Hence, the
altitude must be determined for the base station, and for all others
that show much change in elevation. Within the ordinary range of
growing-period temperatures, the effect of pressure is not great. For
all ordinary cases, it suffices to compute tables for pressures of 30,
29, 27, 25, and 23 inches. The following table indicates the decrease in
pressure which is due to altitude.
ALTITUDE PRESSURE
Feet Meters Inches Centimeters
0 0 30 76
910 277 29 73.5
1850 574 28 71
2820 860 27 68.5
3820 1165 26 66
4850 1477 25 63.5
5910 1792 24 61
7010 2138 23 58.5
8150 2485 22 56
9330 2845 21 53.5
10550 3217 20 51
13170 4016 18 46
16000 4880 16 41
The fluctuations of pressure due to weather are usually so slight that
their influence may be disregarded. An excellent series of tables of
relative humidity is found in Marvin’s Psychrometric Tables, published
by the U. S. Weather Bureau, and to be obtained from the Division of
Publications, Washington, D. C., for 10 cents. A convenient field form
is made by removing the portion containing the tables of relative
humidity, and binding it in stiff oilcloth.
[Illustration: Fig. 10. Conversion scale for temperatures.]
=74. Sums, means, and curves.= An approximate humidity sum can be
obtained by adding the absolute humidities for each of the twenty-four
hours, and expressing the results in grains per cubic foot. It is
possible to establish a general ratio between this sum and the
transpiration sum of the plant, but its value is not great at present.
Means of absolute and of relative humidity are readily determinable from
the psychrograph records; the latter are the most useful. The mean of
relative humidity for the twenty-four hours of a day is the average of
the twenty-four hour humidities. From these means the seasonal mean is
computed in the same manner. A close approximation, usually within 1
degree, may be obtained in either case by averaging the maximum and
minimum for the period concerned. Various kinds of curves are of value
in representing variation in humidity. Obviously, these must be derived
from the psychrograph, or from the psychrometer when the series is
sufficiently complete. The level curve indicates the variation in
different stations at the same time. These may be combined in a series
for the comparison of readings made at various heights in the stations.
The day or point curve shows the fluctuations during the day of one
point, and the station curve the variation at different heights in the
same station. The curves of successive days or of different stations may
of course be combined on the same sheet for comparison. Level and
station curves based upon mean relative humidities are especially
valuable.
=75. Records.= A field form is obviously unnecessary for the
psychrograph. The record sheets constitute both a field and permanent
record. The altitude and other constant features of the station and the
list of species, etc., are entered on the back of the first record
sheet, or, better, they are noted in the permanent formation record. For
psychrometer readings, whether single or in series, the following record
form is employed:
════════╤════╤═════════╤════════╤════════╤═══════════╤═══════╤════╤════
│ │ │ │ │ │Height │Dry │Wet
Date │Hour│Formation│Station │Altitude│ Community │ of │bulb│bulb
│ │ │ │ │ │reading│ │
────────┼────┼─────────┼────────┼────────┼───────────┼───────┼────┼────
„ │ „ │ „ │ „ │ „ │ „ │ „ │ „ │ „
────────┼────┼─────────┼────────┼────────┼───────────┼───────┼────┼────
15/8/’04│6:20│Spruce │Brook │ 2500 m │Mertensiare│ 1 ft. │51° │46°
│A.M.│ │bank │ │ │ │ │
„ │ „ │Half │Hiawatha│ „ │Asterare │ „ │56° │49°
│ │gravel │ │ │ │ │ │
„ │6:45│Spruce │Brook │ „ │Mertensiare│ „ │54° │52°
│P.M.│ │bank │ │ │ │ │
„ │ „ │Half │Hiawatha│ „ │Asterare │ „ │56° │52°
│ │gravel │ │ │ │ │ │
────────┴────┴─────────┴────────┴────────┴───────────┴───────┴────┴────
════════╤════╤═════════╤════════╤═════╤════╤════╤════╤═══════════════
│ │ │ │ │Rel.│Base│Abs.│
Date │Hour│Formation│Station │Diff.│Hum.│Hum.│Hum.│ NOTES
│ │ │ │ │ │ │ │
────────┼────┼─────────┼────────┼─────┼────┼────┼────┼─────┬────┬────
„ │ „ │ „ │ „ │ „ │ „ │ „ │ „ │ Sky │Rain│Wind
────────┼────┼─────────┼────────┼─────┼────┼────┼────┼─────┼────┼────
15/8/’04│6:20│Spruce │Brook │ 5 │72% │63% │2.9 │Clear│ 0 │ 0
│A.M.│ │bank │ │ │ │ │ │ │
„ │ „ │Half │Hiawatha│ 7 │64% │63% │3.0 │ „ │ 0 │ 0
│ │gravel │ │ │ │ │ │ │ │
„ │6:45│Spruce │Brook │ 2 │89% │69% │4.2 │ „ │ 2 │ 0
│P.M.│ │bank │ │ │ │ │ │cc. │
„ │ „ │Half │Hiawatha│ 4 │79% │69% │4.0 │ „ │ 2 │ 0
│ │gravel │ │ │ │ │ │ │cc. │
────────┴────┴─────────┴────────┴─────┴────┴────┴────┴─────┴────┴────
On page 47 is given a table for the conversion of Centigrade into
Fahrenheit temperatures. This may be done mentally by means of the
formula _F_ = _C_/5 × 9 + 32°.
LIGHT
=76. Methods.= All methods for measuring light intensity, which have
been at all satisfactory, are based upon the fact that silver salts
blacken in the light. The first photographic method was proposed by
Bunsen and Roscoe in 1862; this has been taken up by Wiesner and
variously modified. After considerable experiment by the writer,
however, it seemed desirable to abandon all methods which require the
use of “normal paper” and “normal black” and to develop a simpler one.
As space is lacking for a satisfactory discussion of the
Bunsen-Roscoe-Wiesner methods, the reader is referred to the works cited
below.[4] Simple photometers for making light readings simultaneously or
in series were constructed in 1900, and have been in constant use since
that time. An automatic instrument capable of making accurate continuous
records proved to be a more difficult problem. A sunshine recorder was
ultimately found which yields valuable results, and very recently a
recording photometer which promises to be perfectly satisfactory has
been devised. Since the hourly and daily variations of sunlight in the
same habitat are relatively small, automatic photometers are perhaps a
convenience rather than a necessity.
_The Photometer_
[Illustration: Fig. 11. Photometer, showing front and side view.]
=77. Construction.= The simple form of photometer shown in the
illustration is a light-tight metal box with a central wheel upon which
a strip of photographic paper is fastened. This wheel is revolved by the
thumbscrew past an opening 6 mm. square which is closed by means of a
slide working closely between two flanges. At the edge of the opening,
and beneath the slide is a hollow for the reception of a permanent light
standard. The disk of the thumbscrew is graduated into twenty-five
parts, and these are numbered. A line just beneath the opening coincides
with the successive lines on the disk, and indicates the number of the
exposure. The wheel contains twenty-five hollows in which the click
works, thus moving each exposure just beyond the opening. The metal case
is made in two parts, so that the bottom may be readily removed, and the
photographic strip placed in position. The water-photometer is similar
except that the opening is always covered with a transparent strip and
the whole instrument is water-tight. These instruments have been made
especially for measuring light by the C. H. Stoelting Co., 31 W.
Randolph street, Chicago, Ill. The price is $5.
=78. Filling the photometer.= The photographic paper called “solio”
which is made by the Eastman Kodak Company, Rochester, N. Y., has proved
to be much the best for photometric readings. The most convenient size
is that of the 8 × 10 inch sheet, which can be obtained at any supply
house in packages of a dozen sheets for 60 cents. New “emulsions,” i.
e., new lots of paper, are received by the dealers every week, but each
emulsion can be preserved for three to six months without harm if kept
in a cool, light-tight place. Furthermore, all emulsions are made in
exactly the same way, and it has been impossible to detect any
difference in them. To fill the photometer, a strip exactly 6 mm. wide
is cut lengthwise from the 8 × 10 sheet. This must be done in the dark
room, or at night in very weak light. The strip is placed on the wheel,
extreme care being taken not to touch the coated surface, and fixed in
position by forcing the free ends into the slit of the wheel by a piece
of cork 8–9 mm. long. The wheel is replaced in the case, turned until
the zero is opposite the index line, and the instrument is ready for
use.
=79. Making readings.= An exposure is made by moving the slide quickly
in such a way as to uncover the entire opening, and the standard if the
exposure is to be very short. Care must be taken not to pull the slide
entirely out of the groove, as it will be impossible to replace it with
sufficient quickness. The time of exposure can be determined by any
watch after a little practice. It is somewhat awkward for one person to
manage the slide properly when his attention is fixed upon a second
hand. This is obviated by having one observer handle the watch and
another the photometer, but here the reaction time is a source of
considerable error. The most satisfactory method is to use a stop-watch.
This can be held in the left hand and started and stopped by the index
finger. The photometer is held against it in the right hand in such a
way that the two movements of stopping the watch and closing the slide
may be made at the same instant. The length of exposure is that
necessary to bring the tint of the paper to that of the standard beside
it. A second method which is equally advantageous and sometimes
preferable does away with the permanent standard in the field and the
need for a stop-watch. In this event, the strip is exposed until a
medium color is obtained, since very light or very deep prints are
harder to match. This is later compared with the multiple standard. In
both cases, the date, time of day, station, number of instrument and of
exposure, and the length of the latter in seconds are carefully noted.
The instrument is held with the edge toward the south at the level to be
read, and the opening uppermost in the usual position of the leaf. When
special readings are desired, as for isophotic leaves, reflected light,
etc., the position is naturally changed to correspond. In practice, it
is made an invariable rule to move the strip for the next exposure as
soon as the slide is closed. Otherwise double exposures are liable to
occur. When a strip is completely exposed it is removed in the dark, and
a new one put in place. The former is carefully labeled and dated on the
back, and put away in a light-tight box in a cool place.
[Illustration: Fig. 12. Dawson-Lander sun recorder.]
=80. The Dawson-Lander sun recorder.= “The instrument consists of a
small outer cylinder of copper which revolves with the sun, and through
the side of which is cut a narrow slit to allow the sunshine to impinge
on a strip of sensitive paper, wound round a drum which fits closely
inside the outer cylinder, but is held by a pin so that it can not
rotate. By means of a screw fixed to the lid of the outer cylinder, the
drum holding the sensitive paper is made to travel endwise down the
outer tube, one-eighth of an inch daily, so that a fresh portion of the
sensitive surface is brought into position to receive the record.” The
instrument is driven by an eight-day clock placed in the base below the
drum. The slit is covered by means of a flattened funnel-shaped hood,
and the photographic strip is protected from rain by a perfectly
transparent sheet of celluloid. The detailed structure of the instrument
is shown in figure 12. This instrument may be obtained from Lander and
Smith, Canterbury, England, for $35.
In setting up the sunshine recorder, the axis should be placed in such a
position that the angle which it makes with the base is the same as the
altitude of the place where the observations are made. This is readily
done by loosening the bolts at either side. The drum is removed, the
celluloid sheet unwound by means of the key which holds it in place, the
sensitive strip put in position, and the sheet again wound up. Strips of
a special sensitive paper upon which the hours are indicated are
furnished by the makers of the instrument, but it has been found
preferable to use solio strips in order to facilitate comparison with
the standards. The drum is placed on the axis, and is screwed up until
it just escapes the collar at the top of the spiral. The clock is wound
and started, and the outer cylinder put on so that the proper hour mark
coincides with the index on the front of the base.
As a sunshine recorder, the instrument gives a perfect record, in which
the varying intensities are readily recognizable. Since the cylinder
moves one-half inch in an hour, and the slit is .01 of an inch, the time
of each exposure is 72 seconds. This gives a very deep color on the
solio paper, which results in a serious error in making comparisons with
the standard. On account of the hood, diffuse light is not recorded when
it is too weak to cast a distinct shadow. It seems probable that this
difficulty will be overcome by the use of a flat disk containing the
proper slit, and in this event the instrument will become of especial
value for measuring the diffuse light of layered formations. The
celluloid sheet constitutes a source of error in sunlight on account of
the reflection which it causes. This can be prevented by using the
instrument only on sunny days, when the protection of the sheet can be
dispensed with.
=81. The selagraph.= This instrument is at present under construction,
and can only be described in a general way. In principle it is a simple
photometer operating automatically. It consists of a light-tight box
preferably of metal, which contains an eight-day lever clock. Attached
to the arbor of the latter is a disk 7 inches in diameter bearing on its
circumference a solio strip 1 cm. wide and 59 cm. long. The opening in
the box for exposure is 6 mm. square and is controlled by a photographic
shutter. The latter is constructed so that it may be set for 5, 10, or
20 seconds, since a single period of exposure can not serve for both sun
and shade. The shutter is tripped once every two hours, by means of a
special wheel revolving once a day. Each exposure is 6 mm. square, and
is separated by a small space from the next one. Twelve exposures are
made every 24 hours, and 84 during the week, though, naturally, the
daytime exposures alone are recorded. Comparisons with the multiple
standard are made exactly as in the case of the simple photometer. The
selagraph is made by the C. H. Stoelting Co., Chicago, Illinois.
_Standards_
=82. Use.= The light value of each exposure is determined by reference
to a standard. When the photometer carries a permanent standard, each
exposure is brought to the tint of the latter, and its value is
indicated by the time ratio between them. Thus, if the standard is the
result of a 5–second exposure to full sunlight at meridian, and a
reading which corresponds in color requires 100 seconds in the habitat
concerned, the light of the latter is twenty times weaker or more
diffuse. Usually, the standard is regarded as unity, and light values
figured with reference to it, as .05. With the selagraph such a use of
the standard is impossible, and often, also, with the photometer it is
unnecessary or not desirable. The value of each exposure in such case is
obtained by matching it with a multiple standard, after the entire strip
has been exposed. The further steps are those already indicated. After
the exact tint in the standard has been found, the length of the reading
in seconds is divided by the time of the proper standard, and the result
expressed as above.
=83. Making a standard.= Standards are obtained by exposing the
photometer at meridian on a typically clear day, and in the field where
there is the least dust and smoke. Exception to the latter may be made,
of course, in obtaining standards for plant houses located in cities,
though it is far better to have the same one for both field and control
experiment. Usable standards can be obtained on any bright day at the
base station. Indeed, valuable results are often secured by immediate
successive sun and shade readings in adjacent habitats, where the sun
reading series is the sole standard. Preferably, standards should be
made at the solstices or equinoxes, and at a representative station. The
June solstice is much to be preferred, as it represents the maximum
light values of the year. Lincoln has been taken as the base station for
the plains and mountains. It is desirable, however, that a national or
international station be ultimately selected for this purpose, in order
that light values taken in different parts of the world may be readily
compared.
=84. Kinds of standards.= The base standard is the one taken at Lincoln
(latitude 41° N.) at meridian June 20–22. This is properly the unit to
which all exposures are referred, but it has been found convenient to
employ the Minnehaha standard as the base for the Colorado mountains, in
order to avoid reducing each time. _Relative_ standards are frequently
used for temporary purposes. Thus, in comparing the light intensities of
a series of formations, one to five standards are exposed on the solio
strip before beginning the series of readings. _Proof_ standards are the
exposed solio strips, which fade in the light, and can, in consequence,
be kept only a few weeks without possibility of error. The fading can be
prevented by “toning” the strip, but in this event the exposures must be
fixed in like manner before they can be compared. This process is
inconvenient and time-consuming. It is also open to considerable error,
as the time of treatment, strength of solution, etc., must be exactly
equivalent in all instances. _Permanent_ standards are accurate
water-color copies of the originals obtained by the photometer. These
have the apparent disadvantage of requiring a double comparison or
matching, but after a little practice it is possible to reproduce the
solio tints so that the copy is practically indistinguishable from the
original. The most satisfactory method is to make a long stroke of color
on a pure white paper, since a broad wash is not quite homogeneous, and
then to reject such parts of the stroke as do not match exactly.
Permanent standards fade after a few month’s use, and must be replaced
by parts of the original stroke. _Single_ standards are made by one
exposure, while _multiple_ ones have a series of exposures filling a
whole light strip. These are regularly obtained by making the exposures
from 1–10 seconds respectively, and then increasing the length of each
successive exposure by 2 seconds. Single exposures of 1–5 seconds as
desired usually serve as the basis for permanent standards, but a
multiple standard may also be copied in permanent form. Exposures for
securing standards must be made only under the most favorable
conditions, and the length in seconds must be exact. The use of the
stop-watch is imperative, except where access may be had to an
astronomical clock with a large second hand, which is even more
satisfactory. The length of time necessary for the series desired is
reckoned beforehand, and the exposures begun so that the meridian falls
in the middle of the process.
Single standards are exceedingly convenient in photometer readings, but
they are open to one objection. In the sunshine it is necessary to make
instant decision upon the accuracy of the match, or the exposure becomes
too deep. In the shade where the action is slower, this difficulty is
not felt. For this reason it is usually desirable to check the results
by a multiple standard, and in the case of selagraph records, where the
various exposures show a wide range of tint, light values are obtainable
only by direct comparison with the multiple standard. The exact matching
of exposure and standard requires great accuracy, but with a little
practice this may be done with slight chance of error by merely moving
the exposure along the various tints of the standard until the proper
shade is found. The requisite skill is soon acquired by running over a
strip of exposures several times until the comparisons always yield the
same results for each. The margin of error is practically negligible
when the same person makes all the comparisons, and in the case of two
or three working on the same reading the results diverge little or not
at all. The efficient difference for light is much more of a variable
than is the case with water-content. It has been determined so far only
for a few species, all of which seem to indicate that appreciable
modification in the form or structure of a leaf does not occur until the
reduction in intensity reaches .1 of the meridian sunlight at the June
solstice. The error of comparison is far less than this, and
consequently may be ignored, even in the most painstaking inquiry.
_Readings_
=85. Time.= The intensity of the light incident upon a habitat varies
periodically with the hour and the day, and changes in accord with the
changing conditions of the sky. The light variations on cloudy days can
only be determined by the photometer. While these can not be ignored,
proper comparisons can be instituted only between the readings taken on
normal days of sunshine. The sunlight varies with the altitude of the
sun, i. e., the angle which its rays make with the surface at a given
latitude. This angle reaches a daily maximum at meridian. The yearly
maximum falls on June 22, and the angle decreases in both directions
through the year to a minimum on December 22. At equal distances from
either solstice, the angle is the same, e. g., on March 21 and September
23. At Lincoln (41° N. latitude) the extremes at meridian are 73° and
26°; at Minnehaha (39°) they are 75° and 28°. The extremes for any
latitude may be found by subtracting its distance in degrees north of
the two tropics from 90. Thus, the 50th parallel is 26.5° north of the
tropic of Cancer, and the maximum altitude of the sun at a place upon it
is 63.5°. It is 73.5° north of the tropic of Capricorn, and the minimum
meridional altitude is 16.5°.
The changes in the amount of light due to the altitude of the sun are
produced by the earth’s atmosphere. The absorption of light rays is
greatest near the horizon, where their pathway through the atmosphere is
longest, and it is least at the zenith. The absorption, and,
consequently, the relative intensity of sunlight, can be determined at a
given place for each hour of any sunshiny day by the use of chart 13.
This chart has been constructed for Lincoln, and will serve for all
places within a few degrees of the 40th parallel. The curves which show
the altitude of the sun at the various times of the day and the year
have been constructed by measurements upon the celestial globe. Each
interval between the horizontal lines represents 2 degrees of the sun’s
altitude. The vertical lines indicate time before or after the apparent
noon, the intervals corresponding to 10 minutes. If the relative
intensity at Lincoln on March 12 at 3:00 P.M. is desired, the apparent
noon for this day must first be determined. A glance at the table shows
that the sun crosses the meridian on this day at 9 minutes 53 seconds
past noon at the 90th meridian. The apparent noon at Lincoln is found by
adding 26 minutes 49 seconds, the difference in time between Lincoln and
a point on the 90th meridian. When the sun is fast, the proper number of
minutes is taken from 26 minutes 49 seconds. The apparent noon on March
12 is thus found to fall at 12:37 P.M., and 3:00 P.M. is 2 hours and 23
minutes later. The sun’s altitude is accordingly 36°. If the intensity
of the light which reaches the earth’s surface when the sun is at zenith
is taken as 1, the table of the sun’s altitudes gives the intensity at
3:00 P.M. on March 12 as .85.
For places with a latitude differing by several degrees from that of
Lincoln, it is necessary to construct a new table of altitude curves
from the celestial globe. It is quite possible to make a close
approximation of this from the table given, since the maximum and
minimum meridional altitude, and hence the corresponding light
intensity, can be obtained as indicated above. For Minnehaha, which is
on the 105th meridian, and for other places on standard meridians, i.
e., 60°, 75°, 90°, and 120° W., the table of apparent noon indicates the
number of minutes to be added to 12 noon, standard time, when the sun is
slow, and to be subtracted when the sun is fast. The time at a place
east or west of a standard meridian is respectively faster or slower
than the latter. The exact difference in minutes is obtained from the
difference in longitude by the equation, 15° = 1 hour. Thus, Lincoln,
96° 42′ W. is 6° 42′ west of the standard meridian of 90°; it is
consequently 26 minutes 49 seconds slower, and this time must always be
added to the apparent noon as determined from the chart. At a place east
of a standard meridian, the time difference is, of course, subtracted.
[Illustration: Fig. 13. Chart for the determination of the sun’s
altitude, and the corresponding light intensity.]
The actual differences in the light intensity from hour to hour and day
to day, which are caused by variations in the sun’s altitude, are not as
great as might be expected. For example, the maximum intensity at
Lincoln, June 22, is .98; the minimum meridional intensity December 22
is .73. The extremes on June 22 are .98 and .33 (the latter at 6:00 A.M.
and 6:00 P.M. approximately); between 8:00 A.M. and 4:00 P.M. the range
in intensity is from .90 to .98 merely. On December 22, the greatest
intensity is .52, the least .20 (the latter at 8:00 A.M. and 4:00 P.M.
approximately). If the growing season be taken as beginning with the 1st
of March and closing the 1st of October, the greatest variation in light
intensity at Lincoln within a period of 10 hours with the meridian at
its center (cloudy days excepted) is from .33 to .98. In a period of 8
hours, the extremes are .65 to .98, i. e., the greatest variation, .3,
is far within the efficient difference, which has been put at .9. For
the growing period, then, readings made between 8:00 A.M. and 4:00 P.M.
on normal sunshiny days may be compared directly, without taking into
account the compensation for the sun’s altitude. Until the efficient
difference has been determined for a large number of species, however,
it seems wise to err on the safe side and to compensate for great
differences in time of day or year. In all doubtful cases, the intensity
obtained by the astronomical method should also be checked by
photometric readings. A slight error probably enters in, due to
reflection from the surface of the paper, and to temperature, but this
is negligible.
86. _Table for determining apparent noon_