In this paper it is proposed, first, to consider the definition of growth in organisms; next, to analyze the processes of growth; then, to show what an important part water plays in growth and the significance of this fact for the developmental process in general; and, finally, to discuss the bearing of the new facts brought forward upon previously formulated laws of growth.

Organic growth I shall define as increase in volume. It has been variously defined by others. Thus Huxley has called growth "increase in size," which is essentially the same definition. Sachs defines growth as an increase in volume intimately bound up with change of form ("eine mit Gestaltveränderung innig verknüpfte Volumenzunahme "), and he illustrates the definition by the example of the growth of a sprout from its beginning to its completed form. In this case two phenomena are distinguishable: first, increase in volume, and, secondly, the filling out of the details of form. As Sachs says, these phenomena taken together are generally denominated" development," and it seems to me decidedly advantageous to retain this term with its usual signification and to distinguish the two component processes by the terms growth and differentiation. Pfeffer's definition differs still more widely from mine. He defines growth as change in form in the protoplasmic body ("die gestaltliche Aenderung im Protoplasmakörper "), and he goes on to say that increments of volume and mass are not proper criteria of growth. Pfeffer illustrates this statement by the following example: A plant stem or a cell membrane can be permanently elongated by extension beyond the limits of elasticity, without the volume necessarily increasing, and he apparently means to include such an artificial deformation in his definition of growth. "And," he continues, "under certain circumstances a diminution of volume of a plant segment can indeed occur as a result of growth, when, for example, the elasticity of the wall is increased by growth and water is pressed from the cell until equilibrium is restored"; I doubt,

1 Contributions from the Zoological Laboratory of the Museum of Comparative Zoölogy at Harvard College, E. L. Mark, Director, No. 80.

however, if Pfeffer would say that in this case the cell, as a whole, had grown, but if he would, then his definition is a wide departure from ordinary usage.

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Vines offers a definition intermediate between that of Sachs and that of Pfeffer. 66 By growth," he says, we mean permanent change of form, accompanied usually by increase in bulk." But then he goes on to say, "Nor does an increase even of the organized structures of an organ, that is of the protoplasm and the cell wall, necessarily imply that it is growing. Thus, an increase of the cell wall may take place without any perceptible enlargement of the cell, as, for instance, when a cell wall thickens." But since the thickening of the cell wall is a "permanent change of form," it should be considered by the author a growth process, were not increase in size of the cell, after all, in the author's mind, the most important criterion of its growth. Finally, Frank finds no other criterion for growth than an increase of volume (dependent, however, upon the increase of a particular substance).

Thus, with these different plant physiologists we find the word growth bearing the ideas of increase of volume and of differentiation, then of differentiation alone, and, finally, of increase of volume alone. Returning now to the definition proposed above, we see that growth as mere increase of volume is to be distinguished from development, from differentiation, even from increase in mass, although the latter may often serve as a convenient measure of growth.

In analyzing the processes of growth in organisms we must recognize at the outset that organisms are composed of living matter and formed substance, and that growth may therefore result from the increase in volume of either of these. The living matter in turn is composed of two principal substances: the plasma and the chylema or cell sap; so growth may be due to the increase of either of these substances, may result either from assimilation, or more strictly from the excess of the constructive over the excretory processes of the plasma, or from the taking in of water.

Other writers have analyzed the process of growth in very diverse ways. I cite a few of their conclusions. Huxley says, "growth is the result of a process of molecular intussusception." According to N. J. C. Müller, "all phenomena of growth depend, in last analysis, upon this, that the molecule of the solid substance is introduced into the region of growth." Frank understands by growth that increase

of volume which consists of the apposition or intussusception of new solid molecules of similar matter ("welche auf der An- oder Einlagerung neuer fester Molecule gleichartigen Stoffes beruhen"). These definitions include what I regard as only half of the process of growth.

On the other hand Bütschli has recognized that growth is, in part, due to increase of the chylema, and Driesch distinguishes two kinds of cell growth: (1) passive growth, due to imbibition of water, and (2) active growth, resulting from assimilation. This classification agrees with the one I have proposed, but I think the term passive growth very inapt, since the imbibition of water is as truly an active process as any other vital activity.

Of the three factors involved in growth-increase of formed substance, of plasma, and of chylema- the part played by the last seems to me to have been underestimated. Plant physiologists have been in the best position to acquire the facts. They have recognized in the tip of the plant three growth regions. At the extreme tip of the stem (or radicle) is the region of rapid cell division but comparatively slow growth; next below is the zone exhibiting the Grand Period of growth; and still below is the zone of histological differentiation (Fig. 1). In the first zone growth of plasma is occurring; in the second zone growth of the chylema is chiefly taking place; in the third zone, there is growth of formed substance. The immense. 15mp

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6 7 8 9 10 days. Fig. 1. Curve of daily growth in length of a disc, originally 1 mm. long, and taken immediately behind the vegetation point of a radicle of Phaseolus. It comes to occupy in successive days the three zones referred to in the text. From SACHS, Lectures on Plant Physiology.

preponderance of the growth of the second period (7 days) is an index to the preponderating influence in growth of the imbibition of water. An analysis of the substance of the stem at different levels below the tip reveals the same thing a sudden increase in the amount of water from 73% at the tip to 88% at the first internode (II), reaching a maximum at 93% in the second internode (III), then falling slightly (92.7) to the fifth internode (VI, Fig. 2). The

Fig. 2. Curve showing the percentage of water in successive internodes of Heterocention, from the tip to the fifth. After KRAUS, Festschr. naturf. Ges. Halle, 1879.

experiments and observations upon which these conclusions rest thus agree in assigning the chief role to water in the growth of plants.

With animals experiments on this point were lacking. It seemed to me, therefore, a worthy thing to determine whether animals followed the same law in respect to the preponderating importance of water in growth. Accordingly I set to work to determine the percentage of water in the body of developing tadpoles at different stages. Eggs and embryos were weighed at various ages after removing superficial water. Then they were kept in a desiccator from which air had been pumped and which contained a layer of sulphuric acid to absorb moisture. After repeated weighings a condition was found in which the drying mass lost no more water (constant weight). The total diminution in weight indicated the mass or volume of free water contained in the organism at the beginning of the experiment. Numerous weighings were made during two seasons upon

Amblystoma, toads, and frogs. All series showed the same thing; the most complete series is that given in the following table:

:

These results are graphically represented in Fig. 3. The curve and table show that, exactly as in plants, there is a period of slow

Fig. 3. Graphic representation of last column of Table, showing percentage of water in frog embryos from 1 to 84 days after hatching. Compare with Fig. 2.

growth accompanied by abundant cell division-the earliest stages of the egg. Then follows, after the first few hours, a period of rapid growth due almost exclusively to imbibed water, during which the percentage of water rises from 56 to 96; lastly comes the period of histological differentiation and deposition of formed. substance, during which the amount of dry substance increases