The elements of botany / [Sir Francis Darwin].

  • Francis Darwin
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: The elements of botany / [Sir Francis Darwin]. Source: Wellcome Collection.

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    the cell-wall separate from the protoplasm by pressure applied to the cover-glass of a mounted preparation. The protoplasm is squeezed out of the broken cell (just as the flesh may be squeezed out of'a grape-skin) and the tom empty walls and crushed fragments of protoplasm remain. The cell-wall is a colourless membrane made of a substance called cellulose. Cellulose is a compound of the greatest importance in plant-physiology. It forms a large proportion of the substance of plants, and is the basis of many products of vegetable origin. Cotton wool, which is made from the hairs on the seeds of the cotton-plant is nearly pure cellulose, and the same is true of filter-paper which is manufactured from vegetable cell-walls. In a wooden match are many thousands of cells of which the walls are cellulose1. If such a piece of wood is dipped in strong sulphuric acid or is charred by fire a mass of charcoal is the result. This fact proves that cellulose contains carbon, and as a matter of fact carbon makes up nearly half the weight of this substance. Cellulose also contains hydrogen and oxygen in the proportion in which they exist in water, its formula being C6Hi0Os. This is the same formula as that of starch,—an important fact, as will appear later on. In spite of the identity of formula the two substances have not identical reactions. Starch is characterised by giving a blue or purple colour with iodine. Cellulose is characterised by not giving this test unless it has been previously treated with acid. In the laboratory it is usual to shorten the process by the use of an acid preparation of iodine. The 1 More accurately lignified cellulose'.