Growth of various Water Plants in Culture Solution . 363 
quantity of organic substance added in the above experiments never 
exceeded 184 parts per million, while the total concentration of mineral 
nutrients reached 5,500 parts per million. The effect of the organic matter 
could therefore not be attributed to its nutrient value, and it evidently 
functions as a growth-promoting substance, enabling the plants to make full 
use of the mineral substances supplied. 
These plants would normally obtain such organic substance in the 
water of the ponds, &c., in which they grow, and it is noteworthy that the 
slower the rate of multiplication of the particular plant employed, the 
longer is the period that elapses before it begins to appreciate the deficiency 
of the mineral nutrient solution, and to respond to the addition of organic 
substance. Such a plant as Lemna minor or major , which normally multi- 
plies very rapidly, will exhaust its original supply of these necessary 
growth-promoting substances, and will therefore respond to their addition 
to the nutrient solution, much more quickly than will such a plant as 
Limnobium , which grows and multiplies at a comparatively slow rate. 
The plants chosen in these experiments are such as would permit of 
a fairly accurate estimate of their growth by counting and weighing, but 
similar results have been obtained with other plants, such as Veronica 
beccabunga , and the plants used are sufficiently diverse in nature to warrant 
the conclusion that all water plants require organic substances for their 
proper growth and development. It also cannot be expected that water 
plants differ so fundamentally from all other plants as to stand alone in 
such a requirement, and the author has already shown 1 that plants such 
as wheat seedlings, when the supply of organic material stored in their 
endosperm has been removed, respond very readily to the addition of such 
substances to the nutrient solution. An extension of these experiments 
with land plants, and particularly with cuttings, which have no seed to store 
up a quantity of organic substance, has been in progress for the last two 
years and will be reported in due course. The results of these experiments 
indicate that the same principle — the necessity for organic substance in 
small quantity — applies to ordinary land plants as well as to the water 
plants for which it has been established in the experiments recorded 
above. 
Summary. 
Lemna major , in common with Lemna minor , was found to be quite 
unable to maintain its normal health in water-culture solutions containing 
mineral nutrients only. Immature plants used at the beginning of the 
experiment failed to reach the adult size, while adult plants began to bud, 
but the young plants thus formed never matured. 
1 Bottomley, W. B. : Annals of Botany, vol. xxviii, No. cxi, pp. 531-40 ; Proc, Roy. Soc., B., 
vol. lxxxviii, pp. 237-47 ( 1 9 I 4)- 
B b 
