262 THE RATE OF GROWTH [ch. 



itself for discussion*. But without opening up this large subject, 

 we may say one more passing word on the remarkable fact that 

 certain chemical substances, or certain physiological secretions, 

 have the power of accelerating or of retarding or in some way 

 regulating growth, and of so influencing the morphological features 

 of the organism. 



To begin with there are numerous elements, such as boron, 

 manganese, cobalt, arsenic, which serve to stimulate growth, or 

 whose complete absence impairs or hampers it; just as there are 

 a few others, such as selenium, whose presence in the minutest 

 quantity is injurious or pernicious. The chemistry of the hving 

 body is more complex than we were wont to suppose. 



Lecithin was shewn long ago to have a remarkable power of 

 stimulating growth in animals t, and accelerators of plant-growth, 

 foretold by Sachs, were demonstrated by Bottomley and others J; 

 the several vitamins are either accelerators of growth, or are indis- 

 pensable in order that it may proceed. 



In the little duckweed of our ponds and ditches [Lemna minor) the botanists 

 have found a plant in which growth and multiplication are reduced to very 

 simple terms. For it multiplies by budding, grows a rootlet and two or three 

 leaves, and buds again; it is all young tissue, it carries no dead load; while 

 the sun shines it has no lack of nourishment, and may spread to the limits of 

 the pond. In one of Bottomley's early experiments, duckweed was grown 

 (1) in a "culture solution" without stint of space or food, and (2) in the same, 

 with the addition of a little bacterised peat or "auximone." In both cases the 

 little plant spread freely, as in the first, or Malthusian, phase of a population 

 curve; but the peat greatly accelerated the rate, which was not slow before. 

 Without the auximone the population doubled in nine or ten days, and with 

 it in five or six; but in two months the one was seventy-fold the other ! 



The subject has grown big from small beginnings. We know 

 certain substances, haematin being one, which stimulate the growth 

 of bacteria, and seem to act on them as true catalysts. An obscure 

 but complex body known as "bios" powerfully stimulates the 

 growth of yeast; and the so-called auxins, a name which covers 

 numerous bodies both nitrogenous and non-nitrogenous, serve in 



* For a brief resume of this subject see Morgan's Experimental Zoology, chap. xvi. 



t Hatai, Amer. Joum. Physiology, x, p. 57, 1904; Danilewsky, C.R. cxxi, cxxn, 

 1895-96. 



X W. B. Bottomley, Proc. R.S. (B), lxxxviii, pp. 237-247, 1914, and other 

 papers. O. Haberlandt, Beitr. z. allgem. Botanik, 1921. 



