We will now see how the soft cells which form the pulpy fruit of the strawberry can be changed into the hard timber of the oak or iron-wood tree.
RINGED STRUCTURE.
Wherever a cell is destined to form part of a permanent tissue, it is strengthened by receiving certain additions to its walls. These additions are technically known as “secondary deposit,” and are made in various ways. Sometimes they extend in a thin layer over the whole cell-wall, leaving a number of little holes, which are called “pits,” and earning the name of “pitted structures.” Very frequently the secondary deposit is arranged in a series of rings, an example of which is given in the accompanying [illustration]. This object is taken from the mistletoe. Good examples of the ringed structures may be seen in the anthers of many plants, and in the leaf-stem of the common rhubarb, an example of which is shown in the next [illustration]. Another very common form of secondary deposit is the spiral, which is generally used where strength and elasticity are united. Two examples of the spiral form are given in the [illustration]; the first taken from the lily, and the second from the “rhizome,” or subterranean stem of the water-lily.
RINGED AND SPIRAL STRUCTURES.
Another beautiful form of secondary deposit is seen in the fern root. If the root be cut longitudinally, and the dark hard fibre dissolved carefully out with nitric acid, the deposit will seem to have assumed the shape of a winding staircase, and is then called “scalariform,” or ladder-shaped. Similar structures may be found in asparagus.