GROWTH. 315 GROWTH. The matter of longevity and also of size may also be dependent on the nature of the cells, ilinot states that the most charaL-teristic pecu- liarity of advancing age, of increasing develop- ment" is the growth of protoplasm in proportion to the size of the nucleus. "The possession of a relatively large quantity of protoplasm is a sign of age" ; that is, the amount of protoplasm in the cell is large in maturity and old age as compared with the nucleus. He refers to Biit- schli's investigations, which indicate that in bacteria a nucleus is present and very large, while the protoplasm enveloping it is minimal in amount. This fact should be correlated with the e.traordinary power of proliferation or re- production in these organisms, indicating that a snuill proportion of protoplasm is essential to rapid growth. This presence of a minimum amount of protoplasm is a 'young' character, a characteristic of youth. Certain sea-fishes, as also starfishes, continue to grow for very long periods, and their cells have the 'young' charac- ter, containing very little protoplasm. On the other hand, there are certain types which do not grow bej-ond a definite size, and in these, as for instance among the insects, the cells are charac- terized by having a great deal of protoplasm in proportion to the nucleus. In this connection it should be remembered that 5Iay-flies and dragon-flies in the Carboniferous period were gigantic in size, their wings expanding from ten to twenty inches, and throughout geological history we have cases of types becoming colossal before becoming extinct. Thus overweight and oversize may be a sign of weakness and decrepi- tude — of senescence of the type. As the result of prolonged observation on guinea-pigs, weigh- ing them regularly at fi.xed ages from birth until maturity, Minot shows that even from the time of birth there is a steady loss of vitality; the animal begins, so to speak, to die as soon as it is born. The amount of protoplasm in the cells increases with age. This loss of growth power is equally demonstrated in the case of man, and presumably of all mammals. The passage from youth to old age is called 'senescence' (q.v. ). and the procreation of the J'oung 'rejuve- nation' (q.v. ). Bibliography. H. Spencer, Principles of Bi- cloriij (New York. 2d ed.. 1900) : Minot, "Growth as a Function of Cells." in Proccedinr/s of Boston Korieiy of Xalural Histor;/, vol. xx. (Boston, 1870) ; "On Certain Phenomena of Growing Old," address, American Association for the Advance- iiient of Science (Salem, 1901) ; "Senescence and Kejuvenation." in Journal of Phi/siolof/;/, vol. xx. ( 1S91) ; Ryder. "The Correlations of the Volumes and Surfaces of Organisms." Contrihnfions to the Zoiilorjical Laborntori/. Unircrsit;/ of Pcnnsijl- rania, vol. i., part i. (Philadelphia. 1S9.3) : Ver- worn. General Physiolotii/ (Xcw York. 1899) ; Davenport, "Effect of Chemical and Physical Agents upon Growth," in Experimental Morphol- ogy, part ii. (New York, 1899), and the litera- ture there referred to. GROWTH (in plants). Three ideas are in- volved in the term 'growth' as applied to plants: First, increase in size; second, formation of new material: third, attainment of maturity. Of these three, increase in size is the predominant one: but in special cases of growth, no ap- preciable increase in size occurs, although there Vol. IX.— 21. Fig. 1. Longitudinal section of tlxo (irnwiiiy- iKiiiitof the slioot orinu'sftail {Kiiuisetum arvense}. The iipex is (M-fupied by a tetra- liedrai ceil, wiiicii is continually dividing. Lateral leaves, and ju.st above thcni the oldest lat>- erai growing points, are forming. may be the formation of new material, and even, on attainment of maturity, a rather com- plex structure. Again, organisms may increase in size and attain mature form without the for- mation of any new living material, the whole course of de- velopment depending merely upon the ab- sorption of water, and the rearrange- ment of material al- ready present. The plant passes through three phases of growth, which are by no means sharply distinguishable from one another, and are established merely for convenience. The Foriiati'e 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 con- tinue dividing, until the plant is construct- ed of a mass of cells. These may remain practically alike, or they may dift'erentiate into unlike groups of cells, each group con- stituting a tissue. (See Histology.) In this case its life his- tory will be more com- 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 aa'e able to ])roduce 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 stace are to be found at the ends of the axes, at which place they constitute the so-called growing Fig. 2. Longitudinal Bection through the growing point of the shoot of a seedling dico- tyl, A group of cells, instead of one, as in equisetum (Fig. 1), occupies the apex. Fig. 3. Longitudinal sec- tion of growing jn'int of rootfroni sfl.igiiiHlla. The triangular t-ell near the tip is the apical cell, divid- ing continuail.v to form (behiml) the tissues of the root-shaft, and (before) the root-cap.
