GROWTH. 316 GROWTH. point (Figs. 1, 2, 3). In the seed-plants, the tip of eaeh rootlet and the tip of each liraneh is occupied by such a region of formative tissue. Even cells which have passed into a later stage of growth may return under certain conditions to the formative stage. This occurs normally in those stems which in- crease in thickness and form one or more growing zones concentric with the surface. A division of the cells in these zones is mainly in a plane parallel to the svirf ace ; the additions increase the diameter of the axes. When a plant is wounded, cells adjacent to those injured may regain their power of di- vision and produce a callus, which closes and heals the wound. Em.ARGEMENr. In the second phase of growth, enlargement, division of the cells has practically ceased, and the greater part of the increase in size is due to the rapid absorption of w^ater, which is secreted by the protoidasm in the form of drops in its substance. As these increase in number and volume, they often unite, and usually by tha time increase in size has been completed' all have united to form one large sap- cavity (vacuole), in which the protoplasm con- stitutes a layer pressed firmly against the cell- wall (Fig. 5)". Since this mode of growth involves Fig. i. Longitudinal eection o( growing* point of root of corn (monocot.vl). Apex (where cell- rows converge) occupied by a group of cella, instead of one, as in eelaginella (Fig. 2). The root-cap is well developed. Fig. 5. a. Several .voung cells from the cortex of a root of the bean ; b. one from the same region after attaining its full size. Drawn to same scale. "The volume of h is more than 1700 times that of a, though, as the wrinkling ehows. it is not turgid. a relative imimportant increa.se in the amount of living protoplasm, it is extremely economical. In this feature the growth of plants differs most strikingly ( in degree at least ) from that of animals. During this second phase of growth the volume of the cells may increase a thou- sand to fifteen hundred fold. Characteristics of the cells in this phase are, therefore, (a) rapid increase in volume, and (b) high percentage of water. The rate of enlargement during this phase is not uniform ; nor does it persist indefi- nitely, even though the conditions may be con- stantly favorable. The increase in volume, at first relatively slow, becomes more and more rapid until it attains a maximum rate, from which point it rapidly declines, and soon ceases altogether. The total period of enlargement is spoken of as the grand period of growth (Fig. 6) . ~ F F ~ n — — ~ n zz — c; — ? — 1 — 2 1 1 ^ 2 1 B ^ s E rfn — E s. = = = E E ^ E = § P^J Fig. 6. DIAGRAM SHOWING PERIODIC VARIATIONS IS GROWTH. The horizontal lines show equal increments of growth; the vertical lines equal intervals of time. a. grand period of growth in root of beau (Vicia); h. daily period of growth in stem of barley; c. daily period of stem of mustard (solid line) and of leaf of onion (broken line). In band c the shaded bars represent hours of darkness. It lasts in diflferent plants from five to fifteen days, depending on various factors which influ- ence both duration and rate. Since the forma- tive regions are located at the tips of the axes, the regions of enlargement may be found a short distance from the tip. In roots the elongating region is very short, comprising usually less than 10 millimeters (O.-t inch). In stems, however, the elongating region is usually from 10 to 20 centimeters in leng-th (4 to 8 inches), and in climbers mav even reach 50 to 80 centimeters (20 to 30 inches). The rate of enlargement (especially of elonga- tion) is likely to be difl'erent on dillcrent sides of a bifacial or radial organ. In that case these organs will be correspondingly curved. The region elongating most rapidly will become con- vex and the opposite side concave. This curva- ture, which is determined by unknown, possibly internal, causes, is called nutation (q.v.). The curvatures of bifacial organs, which are also dorsiventral, arc further distinguisliod by the terms 'epinasty' and 'hyponasty.' An epinastic curvature is the efl"ect of greater growth upon the dorsal side; a hyponastic curvature upon
