Because, one of the chief functions of the bark in exogenous trees, is to _protect the sap_ from which the wood is formed on the outward surface; and as there is no such external flow of sap in endogenous trees, the bark is _unnecessary to them_, and is therefore withheld. They are furnished instead with a thin cuticle. 1283. _Why do endogenous stems grow to a great height?_ Because, as the stem grows from the centre, it soon reaches that limit of diameter _which its vascular structure is calculated to support_; and, therefore, the wood-forming sap is deposited chiefly at the top of the stem, causing it to grow to a considerable height. 1284. _Why do the various vegetable fruits ripen in succession?_ Because the Author of Nature has thus arranged its economy, _in order that the wants of living creatures may be adequately provided for_. Some vegetable productions arrive at their perfection in the spring; others in summer; and others in autumn. Among the latter are many that require to come slowly to maturity after they are gathered; by these the winter season is provided for, and a surplus of the winter stock goes to supply the natural deficiency of spring. [Verse: "O sing unto the Lord a new made song; for he hath done marvellous things."--PSALM XCVIII.] 1285. _Why, when seeds are sown, and germination begins, does the leaf-germ seek the light, and the root-germ grow down into the earth?_ Because the Creator has endowed every single seed with a _vital instinct which governs its development_. The rootlet could more easily grow upward than downward, because of the looser earth, and of the exciting influences of light and moisture. Yet it takes the contrary course, leaving the leaf-germ to come up to meet the sun-light, and to send down to the stem and roots, the matter needed for their growth. Frequently, indeed, when seeds are thrown into the earth, their natural position is reversed, and when the germs first start from the seed, the _root-germ_ is directed _upward_ and the _leaf-germ downward_. What then occurs? They each turn, and, in doing so, frequently cross each other. Each goes to its particular duty--the duty that God appointed. CHAPTER LXV. 1286. _Why are the seeds of plants indigestible?_ Because they are encased in a hard covering upon which the gastric juice of animals takes no effect. This provision has been made by the Creator, _for the preservation of seeds_, the productions of which are so essential to animal life. The gastric juice can dissolve any other part of the plant, even the woody fibre, and yet upon the _seed_ it takes no effect. When, however, the seed is _crushed_, and, thereby, the vital principle destroyed, so that no plant can spring from it, the gastric juice acts upon it, and it is soon dissolved. Hence graminivorous birds are provided with gizzards _to break the protecting coats of the grain_; and animals that feed on seeds and nuts _strip them of their shells and husks_.