Anatomy of the Vegetative Organs of the Parsnip

Abstract

1. The mature root of the parsnip represents the true primary root together with the hypocotyl. A longitudinal section shows that the major portion is composed of phloem parenchyma whose cells are filled with starch grains. The central portion of the root is occupied by a core of xylem, and in the hypocotyl pith lies within the xylem. Coarse leaves are diverged from the foreshortened stem during the first year and a coarse columnar axis bearing leaves, flowers, and fruit is developed during the second year. 2. The primary tissues of the root consist of xylem differentiated as a diarch and two groups of phloem radially arranged and separated by interstitial parenchyma, also a pericycle, endodermis, cortex, and epidermis. The hypocotyl consists of the same tissues except that pith is differentiated at the center and separates the xylem. 3. Oil ducts are differentiated in the pericycle and primary phloem, and are relatively ephemeral. 4. A very small number of plants appeared in which the hypocotyl seemed to be triarch and in which triple cotyledons were developed, but it was determined that the third arch resulted from the splitting of one of the bundles while in transition; and since the associated roots were diarch in every case and various irregularities in structure were noted in each of these plants, they cannot be considered true triarchs. 5. Permanent and transient secondary roots are initiated in the pericycle, and other groups of laterals are differentiated in the phellogen near the free conductive tissue of transients which have sloughed off. 6. The foreshortened stem differs little in structure from the hypocotyl, except that the pith is broader and the leaf traces form a band of bundles. 7. The transition commences in the lower part of the hypocotyl, and the bundles have been reoriented to a tangential position (or 90⚬ from the original central position) at the level where the cotyledons diverge from the stem and the reorientation from tangential to true endarch occurs in the petiole of the cotyledons. Lateral cotyledonary traces accompany the transition bundles above the cotyledonary plate. 8. The large coarse leaves vary greatly in texture, color, and the number of leaflets involved. The petiole consists of an epidermis, collenchyma, photosynthetic and cortical parenchyma, and bundles which are normally collateral but which separate, reorient, and anastomose frequently, and in these processes various types of bundles are evolved. The blade is composed of an epidermis whose cells are of two types, tabular and tortuous, also palisade cells and spongy mesophyll. 9. Hairs and striations are developed on the ridges of the petiole and floral axis, and on the veins of leaves. Stomata appear on the stem, in the grooves of the petiole and axis and tortuous cells of the leaves. 10. The floral axis is initiated during the first year and develops rapidly during the second. It consists of an epidermis, collenchyma, photosynthetic and cortical parenchyma, a band of bundles, and a central pith which develops a schizogenous split at the internodes. Flowers and fruit develop during the second year. 11. Secondary thickening involves the activity of a cambium, and in the root and hypocotyl the tissues centripetal to the pericycle are sloughed off soon after secondary growth commences and a very regular periderm is developed by the proliferation of pericyclic cells. In the other organs much of the growth is due to cell enlargement, although in the bundles there is a limited amount of cambial activity. 12. The greater portion of the root and hypocotyl remains in a parenchymatous state during the first year, and increase in size is accomplished through the enlargement of individual cells. The primary xylem cells are forced apart by the growth of the xylem parenchyma and the protoxylem is gradually resorbed, while the metaxylem is scattered through the parenchyma and early secondary xylem, from which it is practically indistinguishable.