694 



TRANSACTIONS OF SECTION K. 



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 strictly uniform acceleration, doubling the weight 

 of the plant every ten days (fig. 3). The precise external conditions are not 

 stated. 



In spite of the morphological complexity the autocatalytic reaction of growth 

 is apparently not checked by inadequate 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 interrupted, 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 diffo-ont organic and mineral 

 constituents worked out for barley and buclvwheat at Ueue\ a are of similar form, 

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

 total fresh weight. 



/uJ 



In this connection the tall and dwarf forms of the same plant present an 

 interesting problem, and some experiments have been started on sweet peas at 

 Cambridge. At the time of germination the seedlings weigh about the same, 

 Avhereas 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 'i Construction of 

 the curves of growth 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. 



Oi 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, for instance, played any part in metabolism which involved 

 its being used up in any building material or by-product of metabolism, then a 



' Stefanowska, Comptes rendus de I'Acad. des Sciences, February 1, 1904 



