Outlines of botany : including a description of mosses, lichens, fungi, ferns, and seaweeds / by J. Scoffern.

  • John Scoffern
Date:
1860
Catalogue details

Licence: Public Domain Mark

Credit: Outlines of botany : including a description of mosses, lichens, fungi, ferns, and seaweeds / by J. Scoffern. Source: Wellcome Collection.

    25/312 (page 15)
    tion still further, and say that phoenogamoiis plants admit of division into exogenous and endogenous ones. This division is quite natural, even if vpe have regard merely to the structure of the stem ; but the agreement is much wider than this, and recognisable by other analogies, as we shall presently see, when we come to consider the nature and peculiarities of leaves and seeds. § 5. CONCERNING LEAYES AND THEIR USES. There are two methods of teaching the nature of a thing ; one is by definition, the other by example. Of these the latter is usually the more easy, but the former is the more precise. Accordingly^ then, we shall com- mence by stating that in botanical language a leaf admits of definition as “ a thin flattened expansion of epidermis, containing between its two layers vascular and cellular tissue, nerves, and veins, and performing the functions of exhalation and respiration.” Such is the botanical deflni- tion of a leaf. Probably the learner may not understand this definition just yet, but a little contemplation will enable him to do so. With the object of enabling him to understand the definition, suppose we go through its clauses one by one. Firstly, then, a thin flattened expansion oj epidermis^ we assume to be a self-evident expression. The epidermis {iTnoepfiig) means, as we have already stated, the outside bark—at least, this is its botanical acceptation. Literally, the Greek word kTriSepflg means skin, and is also applied to indicate that portion of the animal skin which readily jieels off, which rises under the action of a blister, and which, when thickened and hardened, constitutes those troublesome pests, corns. As regards the epidermis of vegetables, it may readily be seen in the birch tree, from which it peels off in long strips. Well, a leaf, then, consists of two layers of this epidermis, one above and the other below, enclosing vascular and cellular tissue, the meaning of which terms we have now to explain to the reader. By vascular tissue is meant those little pipes or tubes which run through vegetables, just like arteries and veins through animal bodies, and which serve the pur- pose of conveying juices from one part of a plant to another. In plants, these pipes or tubes are so exceedingly small that their tubular character is only recognisable by the aid of a microscope or powerful lens, but their presence may be recognised by the naked eye. Thus, for example, we have little doubt that most readers of this work have noticed that, on breaking across a portion of succulent vegetable stem, such, for in- stance, as a piece ot pie-rhuharh, that the two portions do not always break clean off, but one part remains attached to the other by certain little fibrils. Now, these fibrils are vascular, that is to say, they are tubes, and tubes of various kinds, admitting of distinction amongst them- selves. These distinctions we shall not enter upon here further than 33 * [Botakt2.]