Mineral Solutions and in their Natural Medium . 351 
contained small amounts of nitrates and of phosphates which could serve as 
nutrients. In spite of this restriction of food supplies, the plants remained 
healthy in appearance and slightly increased their original size, this increase 
being probably due to the lack of competition normally met with in their 
natural situation, combined with the more uniform conditions prevailing 
during the experiment. It is therefore presumably the presence of the 
organic matter which enables the plants to maintain their normal health, 
while the lack of food materials acts as a limiting factor restricting their rate 
of multiplication. The absence of this organic matter from the solutions 
probably explains the difficulty experienced by Darwin and Acton 1 in 
growing water plants in culture solutions. 
Summary and Conclusions. 
The experiments here recorded emphasize the necessity for organic 
matter for the optimum growth and development of Lemna minor . Previous 
work on the part of the author has shown that these plants are unable to 
grow healthily for any length of time in Detmer’s nutrient solution, while 
the addition of organic matter both accelerated the growth and promoted 
the health of the plants. 
Experiments carried out with Knop’s solution demonstrate that this is 
no more capable than is Detmer’s medium of maintaining the plants in 
health, while the former solution together with organic substance enables 
the plants to multiply more rapidly .and retain a healthy appearance. 
The failure of the nutrient solution previously employed to satisfy the 
needs of the plants is therefore not due to an unsuitable combination of 
materials, since the widely used Knop’s solution is equally inefficient. 
Plants of Lemna major showed a similar requirement for organic 
material. Young plants used at the beginning of the experiment, when 
grown in mineral nutrients only, even failed to attain the adult size, but 
began to multiply in the immature condition, while the addition of organic 
substance had the effect of enabling the plants to rapidly become full grown, 
at the same time showing an increased rate of multiplication. 
The growth of plants of Lemna minor in mineral solutions was com- 
pared with that in the water of the pond in which they were originally 
growing, with the result that though in the latter medium the plants did not 
multiply so rapidly, they retained and even slightly increased upon their 
original size and remained quite healthy. 
Analysis of the pond water showed an average content of 1 ,577 parts 
per million of total dissolved solids, of which 488 parts were organic. The 
inorganic constituents included very small quantities of nitrates and 
phosphates. 
1 Darwin and Acton: Practical Physiology of Plants, Cambridge, 1901, pp. 61-3. 
