Showing Root and Leaf Growth

    These two streams of sap, one going up and the other returning to the roots, each in its proper channel, are interspersed with air chambers that extend from the center of the tree out toward the bark, where they end in inconspicuous dots called lenticels. It is as though nature had provided an air-cooling device for the constant activity of these diverse currents. These lenticels are prominent on the bark of cherry, but whether obvious or not they are found in nearly all woody stems and insure a constant supply of fresh air to the busy interior.

    In palms, sugar cane, corn, bamboo, and many other plants there is not any distinction between heartwood and sapwood, and in place of bark there is nothing but an outer rind, harder than the interior tissue. Such stems do not usually rot first at the center, have no cambium, and have no annual rings. This method of growth and structure is associated nearly always with definite leaf and flower forms peculiar to it and differing from most other plants. So fundamental are these characteristics, so uniform their occurrence and so clear are the distinctions between them and other plants that botanists have divided all flowering plants into those belonging to this group or to some others. More will be said of this in the chapter on the Families of Plants and Their Relationship.

    For purposes of study, these compounds may conveniently be divided into three groups; namely, the natural gums and pentosans, the pectins and mucilages, and the celluloses. The segregation into these three groups is not sharply defined. The distinction between the groups is based upon the solubility of the compounds in water. The gums and pentosans readily dissolve in water; the pectins form colloidal solutions which are easily converted into "jellies"; the mucilages do not dissolve but form slimy masses; while the celluloses are insoluble in and unaltered by water. Some authors add a fourth group, known as "humins"; but as these are the products of decay (usually in the soil) of these structural compounds, rather than of growth and development, they need not be taken into consideration in a study of the chemistry of plant growth.

    So the palmate leaf, if again compounded in the same way, becomes twice palmate, or, as we say when the divisions are in threes, twice ternate (in Latin form biternate); if a third time compounded, thrice ternate or triternate. But if the division goes still further, or if the degree is variable, we simply say that the leaf is decompound; either palmately or pinnately decompound, as the case may be.