October i, 190S] 



NA TURE 



561 



diminishes considerably. This difference from a simple 

 alga or bacterium we can only regard as a secondary 

 acquisition in relation to the external conditions — either a re- 

 action to a present external stimulus or to the memory of past 

 ■stimuli. In a flowering plant, too, all the cells do not con- 

 tinue to grow; many cells differentiate and cease to grow, 

 and also some of the groups of meristem remain dormant 

 in axillary buds. Clearly the growth curve cannot con- 

 tinue to accelerate logarithmically, and in later phases it 

 must tail off; the "grand period" which growth is said 

 to exhibit is another way of stating this. It will, how- 

 ever, be of great interest to us to see what will be the 

 form of the curve of growth during the early period of 

 development. 



The importance of this class of work has been realised 

 in Geneva, and detailed work is now being done under 

 the inspiration of Prof. Chodat ' in which the curve not 

 only of growth (fresh weight) but of the uptake of all the 

 separate important elements in selected plants is being 

 carefully followed. 



With plants grown in the open, climatic disturbances 

 must occur. We shall therefore figure a curve for the 

 fresh weight of a maize plant grown in water-culture. This 

 is prior to the Geneva work, and due to Mile. Stefanowska,' 

 who has studied also the growth-curves of small animals. 

 The first phase of the curve, lasting some fifty days, shows 

 strictiv uniform acceleration, doubling the weight of the 



J>a^s ° 



4.0 



60 70 50 9" 



plant every ten days (Fig. 3). The precise external con- 

 ditions are not stated. 



In spite of the morphological complexity the autocata- 

 lytic reaction of growth is apparently not checked by in- 

 adequate supplies before the plant enters rather suddenly 

 upon the second phase. Here, from the present point of 

 view, we consider that the progress of growth is inter- 

 rupted, not by the primary physical-chemical causes, but 

 by secondary causes, presumably to be classed in the 

 category of stimulus and reaction. 



The numerous curves for the accumulation of different 

 organic and mineral constituents worked out for barley 

 and buckwheat at Geneva are of similar form, but do 

 not keep up the uniform rate of doubling so well as does 

 the curve of total fresh weight. 



In this connection the tall and dw.irf forms of the same 

 plant present an interesting problem, and some experi- 

 ments have been started on sweet peas at Cambridge. At 

 the time of germination the seedlings weigh about the 

 same, whereas at the end of the season the weight of a 

 tall plant is many times that of a dwarf " cupid " growing 

 alongside under similar conditions. Is the difference due 

 to a less vigorous autocatalysis in the dwarf form, so that 

 throughout its growth it takes a greater number of days 

 to double its weight? Construction of the curves of growth 



^ A. Monnier, " Les Matieres min^rales et la Loi d'Accroissement des 

 Vege'taux," Geneva. 1905 : N. De'l&no. " Le Role et la Fonction des Sek 

 mine'raux dans la Vie de la Plante," Geneva, 1907. .See also the independent 

 work of Tribot, Comptes rettdtis de I'Acad- des Seiciices, October 14, 1907. 



2 Stefanowska, Comptes rendus dc r Acad, des Sciences, February i. 1904. 



NO. 2031, VOL. 78] 



through the season will show whether it is this or some 

 other alteration in the form of the curve. 



I now propose to say a few words about one last point 

 in connection with growth considered as a phenomenon 

 of catalysis before passing on to deal with the effects of 

 temperature. 



Of the metallic elements that are essential for the growth 

 of plants some occur in such minute quantities that one 

 can only imagine their function is catalytic. If iron, tor 

 instance, played any part in metabolism which involved 

 its being used up in any building material or by-product 

 of metabolism, then a larger amount than actually suffices 

 should be advantageous. If its function is catalytic the 

 iron would go on acting indefinitely without being con- 

 sumed, and so a minute trace might serve to carry out 

 some essential, and even considerable, subsection of meta- 

 bolism. 



Elements like manganese, magnesium, and iron are often 

 associated with non-vital catalytic action, and a prepara- 

 tion of iron has recently been quantitatively investigated 

 which seems to have literally all the properties of an 

 organic oxydase from plant tissues.' 



.So long ago as 1869 Raulin observed that traces of 

 unessential salts, in particular those of zinc, added to 

 the culture medium in which he grew the fungus Sterig- 

 matocystis caused a rapid acceleration of the growth rate. 

 The time that the mycelium took to double its weight was 

 now reduced to a half or even a third. This continued 

 enormous effect of so small a trace of substance is possibly 

 to be regarded as an addi^d catalyst to the normal proto- 

 plasmic apparatus. This sort of effect is currently labelled 

 *' chemical stimulation," and has been interpreted as an 



/vfiA^ 



■S -4- -s" 



attempt of the fungus to grow away from an unpleasant 

 environment. To me it looks as if such chemical stimula- 

 tion were really another example of the injudicious exten- 

 sion of the concept of stimulus and reaction. 



This effect of zinc upon the growth of mycelium has 

 recently been verified and extended by Javillier," who has 

 made comparative cultures with increasing doses of zinc 

 salt. He grew Sterigmatocystis for four days at 34° C. 

 in media with graded additions of zinc salts. As the 

 graphic representation shows, he finds a continuous regular 

 increase of the number of grams of final dry weight 

 with doses up to 00000 1 per cent., and then no greater 

 but an equal effect up to 100 times as large a dose. 



This form of curve with uniform rise at first, abruptly 

 changing to a level top, suggests, as I have pointed out 

 elsewhere,^ the cutting-off of the primary rising effect by 

 a limiting factor. In this case presumably the limit set 

 by some other subsection of the metabolism has been 

 attained. 



Acceleration of Re.action-velocitv by Temper.\ture. 



We now turn to consider the fourth and last of the prin- 

 ciples of chemical mechanics which we might expect to 

 find manifested in metabolism. 



It is a universal rule that rise of temperature quickens 

 the rate at which a chemical reaction proceeds. Of course, 

 in some rare conditions this may not be obvious, but be 

 obscured by superposed secondary causes ; but almost always 

 this effect is very clearly marked. 



Further, the nature of the acceleration is a peculiar one. 



' J. WoltT, "Des Ptfroxydiastases artificielles," Comptes rendus dc 

 lAcad. des Sciences, Tune 9, 1908. 



- Comptes rendns dc V Acad, des .Sciences, December, 1907. 



3 "Optima and Litniting Factors," .4««(z/j of Botany, vo\. xix., April, 

 1905. 



