With the naked eye, still better with a lens, a slight yellowish swelling is seen at each-tip. This is the root-cap. It fits over the root as a thimble fits over a finger. The analogy is not good, because root-tip and root-cap are intimately united.
The root-cap bears the brunt of friction as the root bores its way through the earth. Its outside layers get worn away as the root pushes on, but new inner layers are formed by actively dividing cells, and so the delicate root-tip is permanently protected.
The whole length of the root is not concerned in this onward growth. The actual region of elongation may be demonstrated by marking off equal intervals on the axis.
This is done with Indian ink. A ruler is put alongside the, and the marks, a millimetre apart, are very carefully made on the root with a mapping pen or a piece of cotton dipped into the ink. The accompanying drawings show the elongation, in this particular case, in a period of twenty-four hours.
The most active growth, as shown by the wide separation of the marks, takes place a little distance behind the root- tip. An inch and a half behind the tip growth in length has ceased, as is seen by the fact that the two intervals here have remained unchanged. The examination of manyshows that root-hairs only appear where growth in length of the root has ceased. This is good economy, for the root-hairs cling firmly to earth-particles, and if they were on the growing region they would be torn away as the root pushed on- wards.
The root-hairs always occupy approximately the samewith reference to the root-tip, so it is evident that they have an ephemeral existence. As they develop on the newly added lengths of root, they die off on the older part behind.
Even to this almost universal rule there are some few exceptions. Two or three members of the Composite, that is, the family to which the Dandelion belongs, not only have root-hairs over their entire length, but these have a fairly permanent existence, continuing their work of absorption for two or three years.