. Introduction to botany. Botany. 6o Introduction to Botany. conducting elements by the process of girdling, for the reason that the bundles are scattered promiscuously throughout the stem (Fig. 20). 42. Summary of Structure and Function. — When we re- view the details in the plan of construction of typical stems, we see that they are so admirably adapted to give strength and facilitate transport that trees can withstand the storms of centuries, and the inter- change of materials between roots and leaves can continue without interruption even in trees which have reached a height of four hundre
. Introduction to botany. Botany. 6o Introduction to Botany. conducting elements by the process of girdling, for the reason that the bundles are scattered promiscuously throughout the stem (Fig. 20). 42. Summary of Structure and Function. — When we re- view the details in the plan of construction of typical stems, we see that they are so admirably adapted to give strength and facilitate transport that trees can withstand the storms of centuries, and the inter- change of materials between roots and leaves can continue without interruption even in trees which have reached a height of four hundred feet. We see that the strengthening elements are laid down ac- cording to approved mechan- ical principles, and that the conducting system is a double highway along which mate- rials may pass in opposite directions without hindering each other in the least degree. We find that the movement of materials radially is provided for by the medullary rays, which extend individually only a short distance longitu- dinally, and so are prevented from transporting materials in any other than the radial direction (see Fig. 22). 43. Transporting Forces. — The forces which are con- cerned in carrying the water upward have not yet been demonstrated with certainty. Atmospheric pressure does not suffice to carry water to the height of tall trees; capil- larity in the tracheal tubes cannot lift the water beyond the height of a middle-sized tree; and osmotic pressure in. Fig. 22. Diagram showing medullary rays from cross, tangential, and radial points of view. The rays are seen at i and h. After Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Stevens, William Chase, 1861-. Boston, D. C. Heath & Co.
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