CHAPTER LXIII.

1254. _What are vegetable gums?_

Vegetable gums are secretions of plants which are generally _soluble in water_, and which subserve various useful purposes. _Gum Arabic_ is one of the most important of this class of vegetable productions.

_Gutta-percha_ is an invaluable substance lately added to the list of known vegetable productions. It is obtained by cutting the bark of trees of the class called _Sapotacea_. Its proper name is gutta Pulo Percha, gutta meaning gum, and Pulo Percha is the island whence it is obtained. But gutta-percha is not, strictly speaking, a gum.

_India-rubber_ is also a vegetable secretion, improperly called elastic gum. It is obtained from the milky juice of various trees and plants, especially from the syringe tree, of Cayenne.

1255. _What are vegetable resins?_

_Vegetable_ resins are derived from the secretions of plants, and are generally distinguished from gums by being _insoluble in water_, but being soluble in spirits.

When one of these substances is soluble in either water or spirits it is called a _gum-resin_.

1256. _What are vegetable acids?_

Vegetable acids are chiefly obtained from _fruit_; but also abundantly from _wood_, by distillation.

[Verse: "Thou art the God that doest wonders."--PSALM LXXVII.]

1257. _What is tannin?_

Tannin is a vegetable production, obtained chiefly from the oak-bark, and from a variety of other vegetable sources. It possesses the peculiar chemical property which renders it valuable in tanning leather.

1258. _What is opium?_

Opium is the produce of the _poppy_, and is obtained from the seed.

1259. _What are vegetable dyes?_

Vegetable dyes are the various colours derived from the secretions of plants, such as _indigo_, _madder_, _logwood_, _alkanet-root_, _&c._

1260. _What is silica?_

Silica is a mineral substance, commonly known as _flint_; and it is one of the wonders of the vegetable tribes, that, although flint is so indestructible that the strongest chemical aid is required for its solution, plants possess the power of _dissolving and secreting_ it. Even so delicate a structure as the wheat straw dissolves silica, and every stalk of wheat is covered with a perfect, but inconceivably thin coating of this substance.

Amid all the wonders of nature which we have had occasion to explain, there is none more startling than that which reveals to our knowledge the fact that a flint stone consists of the mineralised bodies of animals, just as coal consists of masses of mineralised vegetable matter. The animals are believed to have been infusorial animalculæ, coated with silicous shells, as the wheat straw of to-day is clothed with a glassy covering of silica. The skeletons of animalculæ which compose flint may be brought under microscopic examination. Geologists have some difficulty in determining their opinions respecting the relation which these animalculæ bear to the flint stones in which they are found. Whether the animalculæ, in dense masses, form the flint; or whether the flint merely supplies a sepulchre to the countless millions of creatures that, ages ago, enjoyed each a separate and conscious existence, is a problem that may never be solved. And what a problem! The buried plant being disentombed, after having lain for ages in the bowels of the earth, gives us light and warmth; and the animalcule, after a sleep of ages, dissolves into the sap of a plant, and wraps the coat it wore, probably "in the beginning, when God created the heavens and the earth, and when the earth first brought forth living creatures," around the slender stalk of waving corn!

1261. _Why is silica diffused over the stems of wheat, grasses, canes, &c.?_

Because it affords strength, density, and durability, to structures that are very light, and which, but for this beautiful provision, would be exceedingly perishable.

[Verse: "For in this mountain shall the hand of the Lord rest, and Moab shall be trodden down under him, even as straw is trodden down for the dunghill."--ISAIAH XXV.]

1262. _Why is guano a productive manure?_

Because it contains, with other suitable elements, an abundance of the _silicous skeletons of animalculæ_.

1263. _Why does a wheat-crop greatly exhaust the soil?_

Because, as well as the _carbon, and the salts_, which form the straw and the grain, it draws off from the soil a great amount of _silica_.

1264. _Why is straw frequently used as a manure?_

Because it gives back, with other substances, a _considerable proportion of silica_, in that form which adapts it to the use of the succeeding crop.

1265. _Why is the structure of herbaceous plants less consolidated than that of woody plants?_

Because, for the most part, herbaceous plants last only _a single year_; they, therefore, do not require the enduring qualities of plants that have to sustain the influences of the elements for a succession of seasons.

1266. _Why are the stalks of plants of light structure generally cylindrical?_

Because the cylindrical form is stronger than any other; _a hollow cylinder_, with moderately thick walls, _is stronger than a solid rod_, containing the same amount of material.

1267. _Why do the stalks of plants become hollow?_

Because the parallel and perpendicular fibres of the stalk are developed _more rapidly than the horizontal_. The growth of the plant, therefore, consists of a kind of _divergence from the centre_.

1268. _Why are the stomata, or pores of leaves, generally placed on their under surface?_

Because, being placed on the under surface, they are _shaded_ from the action of the _sun's rays_, and so carry on the function of respiration more actively than if subjected to direct heat; they are also protected from the injurious _effects of dust_; and are moistened by _evaporation from the earth's surface_.

[Verse: "The trees of the Lord are full of sap: and the cedars of Lebanon which he hath planted."--PSALM CIV.]

1269. _Why have plants a formation of pith in their centre?_

The pith is the chief organ of nutriment, especially in the young plant. It is the structure which first conveys fluids to, and receives them from, the newly-formed leaf. It communicates with every branch, leaf, bud, and flower; and also with the bark, through the _medullary rays_, which radiate horizontally from the centre of the plant. It is the centre of the movements of the sap which occur in the horizontal vessels; and it holds an important influence over the life of the plant.

1270. _Why are trees covered with bark?_

Because the bark serves to protect the woody structure, and also to give a passage to the descending sap which flows abundantly in the spring, and out of which the woody fibre is formed. It is also, from its peculiar nature, well fitted to endure the changes of the seasons for many years; and from its non-conducting properties it serves to maintain the equal temperature of the vital parts of the tree.

1271. _What is cork?_

Cork is the bark of a description of _oak-tree_, which grows in great abundance in Spain, Italy, and France.

1272. _Why does the cork-tree release its own bark?_

Because it possesses a bark which is exceedingly _useful to man_; and it seems, therefore, to have been the design of providence that the tree should cast it off, to be applied to the wants of the human family; for the cork-tree does not discharge its bark by the mere cracking, or exfoliation, of its substance; the tree retains the bark for a number of years, until it has attained that consistency and thickness which renders it useful, and then the tree forms within the bark a series of tabular cells, which _cut off the connection of the bark with the internal structure_, after which it peels off in large sheets.

[Verse: "And all the trees of the field shall know that I the Lord have brought down the high tree, have exalted the low tree, have dried up the green tree, and have made the dry tree to flourish: I the Lord have spoken, and have done it."--EZEK. XVII.]

Man assists this evident intention of nature, by slitting the bark from the top of the tree to its base; but even were this not done, the bark would be cast off by the tree itself.

Another proof of design in this useful adaptation of the cork-tree is to be found in the fact, that it thrives under treatment that would destroy other trees. The cork-tree will endure the barking process for _seven or eight successive years_.