ARRANGEMENT OF LEAVES 45 



these bundles are composed, are large and numerous. This helps to 

 account for the rapidity of growth of corn under favorable conditions. 



Growth of Stems. An examination of a longitudinal section of a 

 growing corn stem will show that above each node the pith and fibro- 

 vascular bundles are of a darker green color. The pith in the upper 

 part of the internodes shows a pure white color and is often rather 

 dry, while at the base of the internodes the cells are full of sap. These 

 cells, as well as the extreme tip of the stem, constitute the growing 

 points of the cornstalk. The possession of 14 to 20 such points 

 enables a corn stalk to lengthen rapidly during the growing season. 

 As the stems come out of the. ground, their upward course is like the 

 unfolding of a telescope. Such rapid extension gives corn a chance 

 to outdo its competitors, the weeds, in the race for supremacy in the 

 field. Corn has an endogenous stem. Growth in diameter takes place 

 on the inside, rather than by adding layers on the outside, as in the 

 case of exogenous plants, sudi as the oak. 



LEAVES. — Arrangement. The leaves arise from the nodes and 

 for some distance from one node, almost to the next above, surround 

 the stem in the form of a sheath. The edges of this sheath meet on the 

 side opposite the blade, which spreads out from the stem above the 

 next node in the same manner, but exactly on the opposite side. The 

 leaves are arranged alternately and arise on, and conceal, the grooved 

 side of the stem. The leaf sheath is movable on the internode. This 

 allows the leaf to swing back and forth upon the stem without break- 

 ing loose at its base. The leaves appearing at the lower nodes are 

 usually abortive, hence there is not a full leaf for each node on the 

 stem. There are, however, usually 12 to 18 leaves upon a stalk, the 

 number varying with the variety, the season, and the soil. Corn 

 which is thinly planted will have a greater number of leaves than 

 that which is closely planted. 



Structure. At the point where the leaf blade spreads away from 

 the leaf sheath and changes its vertical course for one more horizontal, 

 there appears a hinge. At this point, the fibro-vascular bundles in the 

 blade are closer together and a light colored triangular spot appears. 

 The blade is especially full near its base for several inches along the 

 edge. This waviness is due to the edge growing more rapidly than 

 the midrib. This extra amount of surface allows flexibility, both in 

 lateral and vertical movements. This ligule is very prominent in 

 corn and its need is demonstrated especially well in the western part 

 of the corn belt. 



