. Elementary botany : theoretical and practical. A text-book designed primarily for students of science classes connected with the science and art department of the committee of council on education . Botany. 44 Elementary Botany. 5. A ring of cambium, also interrupted by the passage of the medullary rays. 6. An interrupted ring of xylem or wood. 7. A central pith or medulla. 8. The medullary rays, uniting the pith with the middle bark. (See fig. 65.) In the monocotyledonous stem (fig. 67) there is quite a dif- ferent arrangement. On the exterior there is no differentiated bark ; within, no se
. Elementary botany : theoretical and practical. A text-book designed primarily for students of science classes connected with the science and art department of the committee of council on education . Botany. 44 Elementary Botany. 5. A ring of cambium, also interrupted by the passage of the medullary rays. 6. An interrupted ring of xylem or wood. 7. A central pith or medulla. 8. The medullary rays, uniting the pith with the middle bark. (See fig. 65.) In the monocotyledonous stem (fig. 67) there is quite a dif- ferent arrangement. On the exterior there is no differentiated bark ; within, no separation into pith and medullary rays ; but a number of bundles of procambium scattered amongst the general ground or fundamental tissue. These bundles differ in their de- velopment as well as in their arrangement from those of the dicotyledonous stem. Instead of leaving a layer of vitally active cambium, they are entirely con- verted into xylem and phloem. Dicotyledonous bundles which contain cambium are spoken of as open, whilst those bundles which are destitute of this form- ative tissue, as those of monocotyledons, are known as closed. We must now note a little more fully the structure and functions of the various parts of the dicotyledonous stem. I. The medulla or pith consists entirely of parenchymatous cells, generally dodecahedral in shape, and it forms a cylindrical axis at or towards the centre of the stem. In the earlier stages the cells usually contain a little chlorophyll, and are filled with nutrient substances ; later on they become dry and colourless, and filled with air, and no longer serve any purpose in the life of the plant, so that the stem may be hollowed, all the pith having disappeared, and yet the plant may be living vigorously. The amount of pith varies much in -different plants. In hard-wooded plants, as the Ebony, it is very small; whilst in soft-wooded plants, as the Elder, it is much larger. Again, we often find in many rapidly growing herbaceous p
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