Part 22

The Iridaceæ include the Crocus (Crocus sativa), the Corn-flag (Gladeolus communis), and the Blue-flag (Iris Germanica); the bulb of the Iris Florentina, is dried and used for various purposes, it has a scent resembling violets and is sold at druggists' shops by the name of orris-root, a corruption of iris-root.

9. ORCHIDACEÆ (Orchids).

The family of the Orchis. Dr. Baird, in describing them, says:--"They are almost all herbaceous, a very few only being somewhat shrubby in their growth; some live in the ground, and besides the ordinary roots have bulbs or starch-bearing tubercles; others are what may be called pseudo-parasites, living attached to the bark of trees. These plants abound in the forests of tropical countries, where the climate is moist, and are generally known by the name of Epiphytes. The flowers vary very much in shape, form, and colour, and in many instances have a striking resemblance to insects, various birds, and animals, as Oncidium, in which the resemblance is to butterflies, &c.; Cychnoches, in which (in one species) the likeness is strikingly similar to a swan; Peristeria, one species of which is called the Sprito-Santo plant, of Panama, and in whose flower there is the likeness of a dove descending upon the lip, &c. A curious fact in this part of their history is, that in the same plant, on the same stem, and even on the same head of flowers, we find flowers so different in appearance that we might place them in different genera."

10. NAIADACEÆ (Naiads).

Water-plants, called "pond-weeds," and grow on both fresh and salt water. The marine species, Zostera marina, is dried and used for stuffing mattresses.

11. BUTOMACEÆ (Flowering Rushes).

Of this order the Butomas umbellatus, a sort of rush, producing very handsome pink flowers in umbels, growing in ditches and by the sides of rivers, is the most characteristic member.

12. ALISMACEÆ (Water Plantain).

The two chief members of this order are the Alisma Plantago or Water Plantain, which grows in ditches, having its flowers in the form of panicles, and the Sagittaria sagittæfolia or Arrowhead.

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The third great natural family of plants are the Exogenæ. They comprise all the trees and shrubs of the temperate and colder regions, together with many of the flowering plants. They are characterised by certain peculiarities which can be at once recognised, such as the twisted and branched form of the stem, the possession of bark, leaves having the veins covering them running in all directions and forming a network, and the seeds containing two cotyledons; the wood, moreover, is deposited in rings (figs. 16, 17, 18, 19), one of which is formed every year, by the new wood being produced on the outside of the old, and between it and the bark. This deposition takes place as follows: after the rains of winter and early spring have well saturated the earth with moisture, and the warmth of spring has begun to penetrate to the roots of the plants, a development of the points of each fibrile or radicle takes place, forming new spongioles; these, being formed of new porous cellular tissue, begin to absorb (by endosmose) the moisture of the earth, which entering at all these thousands of minute spongioles at once, collects and rises in the vessels of the trunk and branches, and arriving at the vessels forming a plexis on the surface of every leaf, begins to be changed by the action of the sun's rays, absorbing carbon and giving out oxygen from the carbonic acid always contained in the air. The sap which has thus risen is the juice of the earth in which the plant grows, containing several earthy salts and vegetable extract drawn from the manure or decaying vegetation contained in the mould, together with carbonic acid dissolved in the fluid; this carbonic acid is changed by the sun's rays as well as that which was contained in the air, and the carbon uniting with the watery part of the sap, forms the green substance before alluded to, called chlorophyll, which is the green colouring matter of all plants, and is the basis of the wood. The altered sap descends between the wood and the bark, and forms a deposit gradually, which at the end of the year is a complete ring of wood surrounding the wood of former years. This circulation of juices continues through the summer, until, the cold weather coming on and the light being diminished, the sap neither rises nor is the leaf nourished by it, when it decays and falls off.

The age of exogenous wood can be ascertained--where the centre has not decayed--by counting the rings, one only being deposited every year; and it is truly astonishing to find that some trees will continue to live and flourish for several thousand years! There does not, in truth, appear to be any limit assigned to the life of an exogenous tree if it escape accidents; for, although decay inevitably attacks the heartwood, and a cavity is the result, yet, the new wood continuing to be deposited on the outer part, the vitality of the tree is kept up, and its size continues to increase.

The Baobab or Monkey Bread-fruit trees, growing at the mouths of the Senegal, have been estimated by Adinson to be upwards of six thousand years old, and are, in all probability, the oldest relics of organic life existing at the present time. The cedars of Lebanon are supposed to have existed longer than the records of history. The Yew at Braburn, in Kent, is at least three thousand years old; and that of Fortingal nearly as much.

Dr. Livingstone, describing the Mowana or Baobab tree (fig. 20), thus comments upon its power of withstanding injury:--

"No external injury, not even a fire, can destroy this tree from without, nor can any injury be done from within, as it is quite common to find it hollow, and I have seen one in which twenty or thirty men could lie down and sleep as in a hut. Nor does cutting down exterminate it, for I saw instances in Angola in which it continued to grow in length, after it was lying on the ground. Those trees, called exogenous, grow by means of successive layers on the outside; the inside may be dead, or even removed altogether, without affecting the life of the tree; this is the case with most of the trees of our climate. The other class is called endogenous, and increases by layers applied to the inside, and when the hollow of the tree is full, the growth is stopped, and the tree must die. Any injury is felt most severely by the first class on the bark, by the second on the inside, while the inside of the exogenous may be removed, or the outside of the endogenous may be cut, without stopping the growth in the least. The Mowana possesses the power of both. The reason is, that each of the laminæ possesses its own independent vitality; in fact, the Baobab is rather a gigantic bulb run up to seed than a tree. The roots, which may often be observed extending along the surface of the ground forty or fifty yards from the trunk, also retain their vitality after the tree is laid low, and the Portuguese now know that the best way to treat them is to let them alone, for they occupy much more room when cut down than when growing."

On examining the wood of the exogenæ, it will be found that in the very centre is a small column of cellular tissue, called the pith, and that from this, fine layers of the same substance radiate towards the circumference. These are called the medullary rays, or silver-grain; they form an exterior layer or ring of cellular substance on the outside of the wood, which is called the cambium. The lightness of wood is owing to its porosity, and, on examining a transverse section under the microscope (figs. 17 and 19), it will be seen how little of real space is occupied in wood by solid substance. The spaces between the wall-work of woody matter in the tree are all filled with sap, and hence it is that "green wood" is so much heavier than that which is well seasoned. It is its lightness and strength, together with the ease with which it can be cut and fashioned, which renders wood so exceedingly useful; but its inflammability and liability to decay are great barriers to its more general use.

The leaves of the exogenæ have their veins always in the form of a network, and not running parallel with each other, as in the endogenæ (fig. 21).

Of the three divisions of the vegetable kingdom, the exogenæ alone furnish building timber, properly so called, and it is doubtful whether nature has supplied a more generally applicable substance than wood; being a bad conductor, it can be handled in the coldest weather, which metal cannot, and, being easily cut or split, can be fashioned into almost any form.

The woods mostly in use for general purposes are the different kinds of pine, as American Pine, Norway Pine, yellow and white Deal, Mahogany, Oak, Beech, Elm, Ash, and Maple. In the "Penny Cyclopædia," under the article "Wood," is the following description of the different kinds used by cabinet-makers, &c.:--

"The woods that are used by the cabinet-maker for furniture of a more delicate kind are called 'fancy woods.' The use of these has become much more general since the introduction of the art of veneering, and now that this is done by machinery instead of by hand, a number of woods are used for furniture and other purposes, which, on account of their scarcity, could have been formerly used only to a very limited extent. The most common of the fancy woods, and that which is most used by cabinet-makers, is Mahogany. This wood is the produce of the 'Swienteria Mahogani.'

"Next in point of importance and use to Mahogany, is Rosewood. This wood obtains its name from its fragrance, and is the produce of a mimosa from the forests of Brazil; in veneering it affords about eight plates to the inch.

"King-wood is a beautiful wood much in use, brought from Brazil in logs four feet long and about five inches wide, and is used only for delicate articles; it is said to be the produce of a species of Baphia, a genus of Leguminous plants.

"Beef-wood is a very heavy wood of a pale red colour, and is brought from New Holland in logs nine feet long and thirteen or fourteen inches wide; the tree from which this is obtained is unknown to botanists, as well as most of those producing the fancy woods of commerce.

"Tulip-wood is brought into the market in very small pieces, not more than four feet long and five inches in diameter, so that probably it is the production of a shrub; it is clouded with red and yellow colours, and is used for bordering and for making small articles, such as caddies and work-boxes.

"Zebra-wood is probably the production of a large tree, as it is cheap enough to be made into tables, pianofortes, &c.; it is coloured brown on a white ground and clouded with black.

"Satin-wood is of a brilliant yellow colour with delicate glowing shades. It is the produce of a plant called Chloroxylon Swienteria, a native of India; it is one of the trees that yield the wood-oil of India, and belongs to the natural order Cedrelaceæ, the same order in which the mahogany is placed; it is found in the market in logs two feet wide and seven or eight feet long.

"Sandal-wood is the produce of a species of Sandoricum belonging to the family Meliaceæ; the wood is of a light brown colour with golden-coloured waves.

"Ebony and Iron-wood are the names given to some very hard woods, the produce of the natural order Ebenaceæ. These woods are mostly brought from India, although some of the species are found in Europe and America.