Growth and decay of the transient (noncambial) roots of alfalfa

The dying of alfalfa rootlets in summer at Madison, Wis., which has been mentioned briefly by the writer previously, has been reexamined as a phase of the obvious, but apparently undescribed, root succession in this plant. Rootlets die following a browning and shrinking of the primary cortex in which the mycorrhizal fungus may or may not be present, and following a soft decay of the ends of rootlets. The agents causing decay are not determined. Two kinds of roots are distinguished in alfalfa on the basis of morphology, duration, and function. They are described as the permanent or the cambial roots and the transient or noncambial roots. The permanent roots, serving for transport and storage, consist almost entirely of secondary growth from a cambium and a phellogen, and are limited to the taproot, a few of its branches, and a few of the branches from the first branches. The transient roots arise in the usual manner from the permanent roots, but they develop little or no cambium and no phellem. When these rootlets die they may be replaced by new transient rootlets emerging near their bases. The rate of transient root growth and regeneration differs greatly during the growing season, and this periodicity of growth is much more marked after the first year. The period of most vigorous root growth begins in May after the stems have begun rapid elongation and reaches its maximum about the first of June. Growth declines by the middle of June, and by the first of July reaches a minimum that persists through the summer. In the latter part of September roots near the surface of the soil become active in growth and continue thus, but with lessened vigor as cold increases in October. No appreciable growth has been found in winter even when the soil is protected from freezing. Root deterioration begins in late May or early June, and increases rapidly from the middle to the end of June. Where decay is severe, the development of rootlets in July and August appears to be largely inhibited by the decay of root ends almost as soon as they emerge. The severity of the decay differs in different fields, and in plants with different habits of growth. It seems possible that the early decay of transient roots limits to some extent the growth of the second crop.