Page:The Oak.djvu/109

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91
THE TREE—ITS ROOT-SYSTEM.

that the substances for making new roots and shoots, new wood, and new leaves, etc., are constructed. It is in the increased area of this leaf laboratory that the larger supplies of salts, dissolved in the larger quantities of water from the soil, are brought into relations with the increased quantities of carbonaceous substance obtained from the air in the chlorophyll corpuscles, and consequently a larger yield of plant-forming materials is possible to meet the demands of the ever-growing organs.

My present purpose is to describe how the thickening process occurs in the older roots, for it is evident at a glance that the strong woody roots of a large tree have undergone many changes since they were the thin filiform rootlets we met with in the young plant (see Fig.7). Not only have they increased in diameter, but they now consist almost entirely of wood, protected by a relatively thin, brown, corky covering, reminding one of certain kinds of bark.

The first changes which take place when the young, thin roots begin to thicken are—first the piliferous layer dies away and the outer cells of the cortex turn brown; then a cylindrical layer of cork is developed in the pericycle, and as this cork is impervious to water it cuts off the cortex from communication with the axis-cylinder, and consequently the cortex gradually shrivels up and is thrown off.

Meanwhile active divisions have been going on in the cells immediately inside the phloëm groups of the