PROXIMATE CONSTITUENTS OF VEGETABLES.
The saccharine, or amylaceous substances constitute the most abundant
of the proximate constituents of plants. They are composed of carbon,
hydrogen, and oxygen. I shall briefly describe the more important
members of this group of substances, namely, starch, sugar, inulin,
gum, pectin, and cellulose.
_Starch_, or _fecula_, occurs largely in dicotyledonous seeds, peas,
&c., and still more abundantly in certain monocotyledonous seeds, such
as wheat and barley. It constitutes the great bulk of many tubers and
roots--for example, the potato and tapioca. It consists of flattened
ovate granules, which vary in size according to the plant. In the
beetroot they are 1/3500 of an inch in diameter, whilst in _tous les
mois_ they are nearly 1/200 of an inch in diameter. Most of the starch
granules are marked by a series of concentric rings. Starch is heavier
than water, and is insoluble in that fluid when cold; neither is it
dissolved by alcohol or ether. When heated in water having a temperature
of at least 140 deg. Fahrenheit, it increases greatly in volume, and
acquires a gelatinous consistence. When the water is allowed to cool,
a portion of the starch becomes insoluble, whilst another portion
remains in solution; the latter form of starch is sometimes termed
_amidin_, from the French word for starch, _amidon_. When dry starch
is heated to 400 deg. Fahr., it is converted, without any change in its
composition, into a soluble gum-like substance, termed _dextrin_,
or British gum. On being boiled in diluted sulphuric acid it is
converted into a kind of sugar; and the same effect is produced by
fermentation--for example, in the germination of seeds. Fresh rice
contains 82, wheat 60, and potatoes 20 per cent. of starch. This
substance constitutes a nutritious and easily digestible food, but
alone cannot support life. Arrowroot is only a pure form of starch.
_Sugar_ occurs less abundantly in plants than starch. There are several
varieties of this substance, of which the kinds termed cane sugar
(_sucrose_) and grape sugar (_glucose_), are only of importance to
agriculturists. The former enters largely into the composition of the
sugar-cane, the beetroot, the sugar-maple, the sorgho grass, pumpkins,
carrots, and a great variety of other plants. Grape sugar is found in
fruits, especially when dried--raisins and figs--in malted corn, and
in honey. In the sugar-cane there is 18 per cent., and in the beetroot
10 per cent. of sugar.
_Cane sugar_, when pure, consists of minute transparent crystals. It is
1-6/10 heavier than water, and is soluble in one-third of its weight
of that fluid. By long-continued boiling in water it is changed into
uncrystallizable sugar, or treacle, by which its flavor is altered, but
its sweetening power increased.
_Grape sugar_ crystallizes in very small cubes, of inferior color as
compared with cane sugar crystals. It dissolves in its own weight of
water, being three times less soluble than sucrose. In sweetening power
one part of cane sugar is equal to 2-1/2 parts of grape sugar; but there
is probably little if any difference, between the nutritive power of the
two substances.
_Inulin_ is a substance somewhat resembling starch. It does not occur
in large quantities. It is met with in the roots of the dandelion,
chicory, and many other plants.
_Gum_ is an abundant constituent of plants. The kind termed gum
arabic, so largely employed in the arts, is a very pure variety of this
substance. Common gums are said to be essentially composed of a very
weak acid--_gummic_, or _arabic_ acid--united with lime and potash.
The solution of gum is very slightly acid, and has a mucilaginous,
ropy consistence: it is almost tasteless. _Mucilage_, or _bassorin_,
is simply a modified form of gum, which, though insoluble in water,
forms a gelatinous mixture with that fluid. It exudes from certain
trees--the cherry for example--and exists largely in linseed and other
seeds. Gums are nutritious foods, but it is probable that they are not
equal in alimental power to equal weights of starch or sugar.
_Vegetable jelly_, or _pectin_, is almost universally diffused
throughout the vegetable kingdom. It is owing to its presence that the
juices of many fruits and roots possess the property of gelatinizing.
It is soluble in water, but prolonged boiling destroys its viscous
property. _Pectose_ is a modification of pectin; it is insoluble in
water. According to Fremy, the hardness of green fruits is due to the
presence of pectose; which is also found in the cellular tissue of
turnips, carrots, and various other roots.
_Cellulose_ is a fibrous or cellular tissue, allied in composition to
starch. It is the most abundant constituent of plants, and forms the
very ground-work of the vegetable mechanism. Linen, cotton, and the
pith of the elder and other trees are nearly pure forms of cellulose.
Ligneous, or woody tissue (_lignin_) is indurated cellulose, hardened
by age. It is almost identical in composition with cellulose. Pure
cellulose is white, colorless, tasteless, insoluble in water, oil,
alcohol, or ether. It is heavier than water. Sulphuric acid is capable
of converting it into grape, or starch sugar. In its fresh and succulent
state cellulose is digestible and nutritious; but in the form of
ligneous tissue it opposes a very great resistance to the action of the
digestive fluids. Digestible cellulose is probably equal in nutritive
power to starch.
_Oils and fats_ occur abundantly in vegetables, more particularly in
their seeds. In the seeds of many cruciferous plants the proportion
of fat and oil exceeds 35 per cent. The oils and fats termed _fixed_
are those which possess the greatest interest to agriculturists; the
_volatile oils_ being those which confer on certain plants their
fragrant odour. There are a great variety of vegetable oils, but
the proximate constituents of most of them are chiefly _stearin_,
_margarin_, _olein_, and _palmitin_.
_Stearin_ is a white crystalline substance, sparingly soluble in alcohol
and ether, but insoluble in water. There are two or three modifications
of this substance, but they do not essentially differ from each other.
The melting point varies from 130 deg. to 160 deg. Fahr. Stearin is the most
abundant of the fats.
_Margarin_ presents the appearance of pearly scales. It is the solid fat
present in olive oil, and it is also met with in a great variety of fats
and oils. It melts at 116 deg. Fahr.
_Olein_ is the fluid constituent of oils and fatty substances. It
resists an extreme degree of cold, without solidifying. There are
several modifications of this body--the olein of olive oil being
somewhat different from that of castor oil; the olein of linseed is
sometimes termed _linolien_.
_Palmitin._--This fat occurs in many plants, but as it makes up the
great bulk of palm oil, it has been termed palmitin. It is white, and
may be obtained in feathery-like masses. Its melting point varies from
114 deg. to 145 deg., there being, according to Duffy, three modifications of
this substance.
The fats and oils are lighter than water. They contain far more carbon
and hydrogen, and less oxygen, than are found in the sugars and
starches. They all consist of acids (stearic, palmitic, &c.) united with
glycerine. On being boiled with potash or soda, the latter take the
place of the glycerine, which is set free, and a _soap_ is produced.
The fatty acids strongly resemble the fats. In nutritive power, one part
of fat is equal to 2-1/2 parts of starch or sugar.
The Albuminous substances contain, in addition to the elements found
in starch, nitrogen, sulphur, and phosphorus. _Albumen_, _fibrin_, and
_legumin_ constitute the three important members of the "Nitrogenous"
constituents of plants.
_Albumen_ is an uncrystallizable substance. It is soluble in water,
unless when heated to 140 deg. Fahr., at which temperature it coagulates,
_i.e._, becomes solid and insoluble. The _gluten_ of wheat is composed
chiefly of albumen, and of bodies closely allied to that substance.
_Fibrin_, when dried, is a hard, horny, yellow, solid body. It contains
a little more oxygen than is found in albumen. This substance is best
known as a constituent of animals, and it does not appear to be abundant
in plants. The portion of the gluten of wheat-flour, which is insoluble
in boiling alcohol, is considered by Liebig and Dumas to be coagulated
fibrin.
In the seeds of leguminous and a few other kinds of plants large
quantities of a substance termed _legumin_ are found. It resembles the
casein, or cheesy ingredient of milk; indeed, some chemists consider it
to be identical in composition with that substance. When pure, it is
pearly white, insoluble in boiling water, but soluble in cold water and
in vinegar. The saline matters found in plants are always associated
with the albuminous bodies; the latter, therefore, form the bones as
well as the muscles of animals.
A great many substances are found in plants, such as wax, mannite,
"extractive matter," citric, malic, and other acids, of the nutritive
value of which very little is known. The substances described in this
section constitute, however, at least 95 per cent. of the weight of the
vegetable matters used as food by live stock.