Histological Studies on the Root of Melilotus alba

Citations Over Time

Abstract

1. The ontogeny of the primary root of Melilotus alba is of the fourth angiospermous type of Janczewski, in which there is a general meristem from which arises the cortex, stele, and central portion of the root cap. The meristem is extended laterally to form the outer parts of the root cap, and at higher levels a dermatogen. 2. Protophloem is the earliest stelar tissue differentiated. 3. Secondary roots arise by cell divisions in the pericycle opposite the protoxylem ridges. The endodermis forms a temporary root cap, which is ultimately crushed or digested, as are also cells of the cortex and epidermis of the primary root by the emerging root. The vascular system is developed in a manner similar to the primary root. 4. Lateral roots develop in the periderm and primary rays of old roots and of the hypocotyl. Cells of the periderm, by radial and tangential divisions, give rise to a temporary sheath over the young root, which is later ruptured. Subsequent development is similar to that of the secondary roots. 5. Secondary tissues develop from a cork cambium which arises in the endodermis, and from a vascular cambium which arises in the seedling between the xylem and phloem. A small amount of tertiary tissue may develop in the pithlike mass of the stele of the tap root late in ontogeny, maturing chiefly as fibers. 6. Parenchyma throughout most of the older roots, especially in the primary rays, becomes active late in the season, separating by its proliferation groups of vessels, fibers, and sieve tubes, crushing the protoxylem, and forming a pithlike mass. It serves as storage tissue for a large reserve of starch. 7. Withdrawal of the hypocotyl underground accompanies proliferation of the parenchymatous tissue and increase in the diameter of the hypocotyl and tap root late in the first season. 8. The hypocotyl has a pith above the level of divergence of cotyledonary traces continuous with that of the stem and present in the seedling. Late in on togeny it develops also a pithlike mass by proliferation of parenchyma in central parts of the stele, which is continuous with that of the root and homologous with it. 9. The root nodule originates in young roots near the close of primary differentiation. Two vascular strands are differentiated which are connected with the stele of the root opposite the nearest protoxylem ridges. An apical meristem adds cells to a central mass of infected and starch-bearing tissues and to a cortex which invests the entire nodule. 10. Root nodules are annual. New ones develop in succession on young roots. 11. Secondary growth continues in the second year through the period of flowering, but at a slower rate. Degeneration occurs after seeds are formed. The older phloem is crushed, the periderm becomes isolated by a layer of dead tissue beneath, most parenchymatous tissue throughout the root develops thick walls, and the cambium ceases to form new tissue. Increase of fibers in the center of the stele is marked. 12. The roots die before the tops; small roots die before the large ones. The tissues finally become disintegrated by the action of saprophytic fungi. Nineteen months is the length of life of the plant from the time of germination of the seed.

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