Substances in Leaf Growth

It was shown by Went7 that leaf growth in etiolated pea seedlings is primarily controlled by substances supplied from the cotyledons. Vein growth is controlled by auxin,2 while growth of the mesophyll is independent of auxin, and it is the factors concerned in the growth of the latter which we will discuss in this paper. In a recent paper3 a bio-assay for these factors was described in detail, as well as a variety of sources, and so will be discussed only briefly here. An outline of the leaf test is as follows: Circular discs ca. 19.5 mm.2 in area are cut from young first foliage leaves of Raphanus. Twenty such discs are floated on the solutions to be tested, and allowed to grow on these solutions for 30 hours at 25°C. The total wet weight of all 20 sections from a single solution is then determined by direct weighing in a standard manner. All solutions to be tested are made up to contain 1% sucrose, and activities are compared to the growth of sections grown in 1% sucrose alone. The source from which a solution of standard activity (S. S. A.) is made is the pea diffusate described by Kogl and Haagen-Smit.6 The present paper will discuss only the activity of various pure substances in increasing mesophyll growth.