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236 CONTRIBUTIONS TO the substances which differ in a chemical sense; and just as little do I require that chemists should approve all my terms and characteristics (independent of this, perfection at the present time would be an impracticable task); I shall merely notice in a few words the most important modifications, their consequence and signification in the course of the development of vegetable organization, in order to avoid repetitions in future.

In the plant starch appears almost to take the place of animal fat. It is superfluous nutritive material, which is deposited for future use; and we therefore usually find it in places where a new formative process is to commence after a short repose, or where a too luxuriant life has generated a superabundance of nutritive material. It has of late been the subject of such deep research that it is unnecessary for me to enter upon it more fully; I will merely refer the reader to the most recent and practical summary of the results in Meyen’s Physiologie, Bd. I, p. 190, &c.

The starch is sometimes supplanted by a semi-granulous substance; for instance, in pollen, the albumen of some plants, and frequently in the cells of the leaf, as matrix of the chlorophylle. It is chiefly distinguished by its occurrence in irregular, granulous forms, which have no internal structure, and from its being coloured a brownish-yellow or brown by tincture of iodine. This substance, which I shall call mucus, is probably identical with that of which the cytoblasts are composed, and with the small granules in gum, which I shall presently mention. Meyen has already remarked the probability of the first supposition (Physiologie, Bd. I, p. 208).

But when the starch is to be employed in new formations, it becomes dissolved, in a manner as yet quite unknown in chemistry, into sugar or gum, the latter sometimes appearing to pass into the former, or "vice versâ". The sugar appears in the form of a perfectly transparent fluid, which is almost as clear as water, is not rendered turbid by alcohol, and receives from tincture of iodine only so much colour as corresponds to the strength or weakness of the solution of the reagent.

The gum appears as a somewhat yellowish, more consistent, and less transparent fluid, which is coagulated into granules by tincture of iodine, assuming a pale yellow permanent colour.