. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. ally 204 SECRETION AND EXCRETION one system are formed also by cell fusions, or sometimes byoutgrowth of branches from the tubes which push their way through intervening tis-sues and fuse with othertubes or branches. In the Urticaceae,Asclepiadaceae, Mora-ceae, most Euphorbia-ceae, and Apocynaceaeeach tube arises from asingle meristematic cellwhich elongates andbranches, keeping pacewith the growth of theplant, and fusing itsbranches with those fromother tubes and thusforming an
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. ally 204 SECRETION AND EXCRETION one system are formed also by cell fusions, or sometimes byoutgrowth of branches from the tubes which push their way through intervening tis-sues and fuse with othertubes or branches. In the Urticaceae,Asclepiadaceae, Mora-ceae, most Euphorbia-ceae, and Apocynaceaeeach tube arises from asingle meristematic cellwhich elongates andbranches, keeping pacewith the growth of theplant, and fusing itsbranches with those fromother tubes and thusforming an intercom-municating system, sothat when a wound ismade the milk poursforth abundantly. The milk tubes re-main living for a longtime and probably takean active part in theproduction of the verycomplex latex or milk,which may containplastic or food sub-stances such as sugar,oil, starch, proteid, and aplastic or non-food substances such as tannins, alkaloids, somevarieties of gum, caoutchouc, resins, and salts of calcium andSome of these may be mere excretions of uselessfrom other tissues, and some of them may be. FIG. n_|.—Laticiferous vessels from the cortexof root of Scorozonora hispanica. A, as seen underlow power, and B, a smaller portion under highpower. (After Sachs.) magnesium,substances TANNIN CELLS 205 products of the tube itself destined for the useful purpose ofhealing wounds and giving immunity from parasitic attacks,while others are clearly foods which find in the tubes an efficientmeans of distribution. Tannin Cells.—Tannin cells are found in various familiesof plants. They occur as single isolated cells or in small cells are approximately isodiametric or in various degreesof elongation. The longest known occur in the genus Sambucus,where they become twenty or more millimeters in length andsometimes extend through an entire internode. Tannin cells are found in the epidermis, primary cortex, peri-cycle, phloem, medullary rays, and in the mesophyll of l
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