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Relatively few investigations have been made to determine the factors which affect the relation of the growth of roots to tops. A more exact knowledge of the relations between aerial and subterranean plant parts and the degree to which these may be modified through cultural practices is of great scientific and practical importance. Extensive investigations have shown that plants exhibit marked specific and varietal differences with respect to relative development of roots when grown under the same environmental conditions. Intensive studies on the growth of wheat by WEAVER, KRAMER, and Ruin (20) and of certain other plants by CRIST and STOUT (3) have also made clear that there is a persistent tendency towards a positive correlation between roots and shoots, increase in size of tops being accompanied by increase in size of roots. Although significant variations in the relative distribution of the growth rate of tops and roots occur in nature and may readily be induced by change in environment, the fact remains that there is a persistent positive correlation in size of tops and roots regardless of the wide variations induced by special conditions.The maintenance of a proper balance between root and shoot is of very great importance. If either is too limited or too great in extent, the other will not thrive. The root system must be sufficiently widespread to absorb enough water and nutrients for the stem and leaves, which, in turn, must manufacture sufficient food for the maintenance of the root system. The plant is a biological unit although it is frequently not treated as such. It is often mutilated by pruning, cutting or injuring the root system, frequently without much regard to the effect upon the remaining portion.JEAN and WEAvER (7), HARRIS (6), and others have shown that the general effect of decreased water content of soil, providing enough is available to promote moderate growth, is to accelerate root development in relation to growth of tops. Low humidity resulting in high transpiration gives similar effects. MAxIMOW and LEBEDINCEV (13)

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PLANT PHYSIOLOGY spectrum. These transformations can be made in the light and without the presence of water in the form of a film surrounding the seed. It appears that under these conditions light does not facilitate the passage of any substance from the seed but causes certain unknown photochemical changes within the seed.-A. L. SHUCK, New York State Agricultural Experiment Station.

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