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The Method by Which Peat-mosses Absorb Water

The tiny openings of the large cells communicate with similar openings in adjoining cells. When the atmosphere is dry, as has already been said, the large cells are filled with air; but as soon as the atmosphere is moist, the surrounding water rushes through the holes on one side of the large cells and the air is forced out through the holes on the opposite sides. When all the large cells are filled with water, the contents of the small cells can absorb through their thin walls the water from the adjoining large cells.

It may help one to comprehend the structure to imagine a number of thin, transparent-glass capsules of irregular shape, with spiral or circular thickenings of glass in their walls and tiny openings here and there. Imagine the capsules piled up with the openings of each capsule contiguous to the openings of adjoining capsules and all the spaces between filled with very small capsules containing a colourless jelly and green granules. If the base of this imaginary pile be immersed in water, immediately the water will creep up through the whole system. This is in accordance with nature's law that water rises in fine hairlike tubes or crevices. To understand the process by which the water passes from a large cell to a small cell one must recall another of nature's laws, that whenever a non-crystallisable substance, as the cell-contents of the small cells, is separated by a membranous partition from a crystallisable liquid, as the water in the large cells, the crystallisable liquid will pass through the membranous wall into the non-crystallisable substance.

It is evident that one function of the large cells is to procure water for the small cells to work with; but since this same function would be performed as well if all the cells were filled with non-crystallisable cell-contents as in other mosses, one must conclude that the large cells serve other purposes beside that of water carriers. Perhaps they are, when filled with air, a protection to the cells containing leaf-green, serving both as shields against excessive heat and light and as a barrier to excessive evaporation.