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How a Hairy-cap Procures a Maximum Amount of Light

The leaves of a Polytrichum have many points of interest.

It is a recognised law in nature that the position of the leaves of a tree or plant is such as to admit the greatest amount of light and air possible to the greatest number of leaves. Since it is the habit of these mosses to grow perpendicularly with little or no branching and to have the leaves long and slender, the leaves are so placed on the stem as to form a spiral of leaves, every eighth leaf lying directly above the first one counted. If a line be started at one leaf, and wound about the stem joining all eight leaves, it will be found that it has coiled three times about the stem. The leaves joined form "one story." If a plant with several stories of eight leaves each has straight perpendicular lines drawn joining leaves which lie one directly above another, it will be found that the distance between each line is 3/8 of the circumference of the stem. The fraction which represents the horizontal distance will always have for a numerator the number of spirals in a story and will always have for the denominator the number of leaves in a story. In some Polytrichum mosses every thirteenth leaf is directly over the first one counted, so that it would require a spiral of five coils to connect all thirteen and would require the circumference to be divided by thirteen perpendicular lines, each line 5/13 of the circumference from the next line. If the two fractions are reduced to the same denominators 40/104 and 39/104 and compared, it will be seen that the leaves were but little more crowded. The extra crowding is compensated for by the greater distance between two succeeding leaves in the same line and by the fact that the leaves in the second instance are narrower than the first.