TILE FORMATIVE OR EMBRYONAL STAGE.
Every plant begins its history as a sin gle cell. The growth and differentiation of this one cell may comprise its whole life history, which will then be correspondingly brief; or, the first cell may divide again and again, and its daughter cells tinue dividing, until the plant is ed of a mass of cells. These may remain practically alike, or they may differentiate into unlike groups of cells, each group stituting a tissue. (See lItsrower.) In this case its life tory will be more plex, and will proceed as follows: At first the cells are all similar in size and form, and each will show these charac teristics: (1) Granular protoplasm without any water-spaces (vacuoles) ; (2) a relatively large nu cleus, its diameter often more than half that of the entire cell; and (3) a relatively thin wall sur rounding the cell. Since this is the condition of the cells in every young (embryo) plant, and since in this condition cells are able to produce new cells by division and to give rise to new organs, this stage is named the em bryonal or formative stage of growth. As the cells grow older, some or all of them pass by im perceptible degrees into the second and third phases of growth, while others remain in the formative stage. Those which persist in this stage are to be found at the ends of the axes, at which place they constitute the so-called growing point (Figs. 1, 2, 3) . In the seed-plants, the tip of each rootlet and the tip of each branch 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 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 surface; the additions increase the diameter of the axes. When a plant is wounded, cells adjacent to those injured may regain their power of vision and produce a callus, which closes and heals the wound.
A It GE MEM'.
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 water, which is secreted by the protoplasm in the form of drops in its substance. As these increase in number and volume, they often unite, and usually by the time increase in size has been completed all have united to form one large cavity (vacuole), in which the protoplasm stitutes a layer pressed firmly against the wall (Fig. 5). Since this mode of growth involves 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).
a relative unimportant increase in the amount of living protoplasm, it is extremely economical. In this feature the growth of plants differs It lasts in different 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 (0.4 inch). In stems, however, the elongating region is usually from 10 to 20 centimeters in length (4 to 8 inches), and in climbers may even reach 50 to 80 centimeters (20 to 30 inches).
The rate of enlargement (especially of elonga tion) is likely to be different on different 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, are further distinguished by the terms `epinasty' and 'hyponasty.' An epinastic curvature is the effect of greater growth upon the dorsal side ; a hyponastic curvature upon the ventral side. Similar variations in growth are induced by the action of external agents; thus, epinasty produced by the action of the light is designated photo-epinasty, etc. Nutation is exhibited by young leaves of ferns, which are rolled upon themselves in a coil and straighten as they grow older; by the leaves of buds and flowers, whose closed form (bud) is due to the more rapid growth of the outer faces and their `opening' to the more rapid growth of the inner faces. The rate of enlargement is affected in various ways by external stimuli (see IRRITA BILITY ) , which will be considered later. Ex amples of very rapid growth are furnished by the filaments of the stamens of wheat, which elongate, as the flower opens, at the rate of 1.8 millimeters (0.07 inch) per minute (about the speed of the minute-hand of a man's watch) ; by the leaf-sheath of the banana, which grows at the rate of 1.1 millimeters (.04 inch) per minute; by the flower-axis of the century-plant, which grows at the rate of 15 centimeters (6 inches) in twenty-four hours; and by the leaves of Vic toria regia, which increase in diameter at the rate of 30 centimeters (12 inches) in twenty four hours. The rate of growth is studied by means of the auxanometer