Growth of various Water Plants in Culture Solution. 357 
pressed plants are shown in PI. XVII, Figs. 1 and 2. The plants in the 
remaining dishes were used for determinations of the dry weight of the 
plants, with the following results : 
100 plants at the beginning of the experiment in both series 
weighed 38-4 mg. 
100 plants at the fourth week in Series I „ 22*0 „ 
» » j) H >) 5**9 >j 
Even though these plants were fully grown to begin with, their off- 
spring were clearly .unable to attain their normal size in the mineral 
nutrients, while those supplied with organic matter appreciably increased 
upon the weight of the original plants. The yellowish colour of the control 
plants was in such marked contrast to the rich green of those in the organic 
substance that it was feared that the nutrient solution might be lacking in 
sufficient quantity of iron. An extra quantity of iron was therefore added 
to the control series, but it had absolutely no effect on either colour or size 
of the plants. 
It is clear from these experiments that certain organic substances are 
quite as necessary for Lemna major as for Lemna minor , and that without 
such materials the plants rapidly become unhealthy, their individual weight 
being diminished and their rate of multiplication retarded. 
Experiments with Salvinia natans. 
Somewhat late in the season, towards the end of August, some experi- 
ments were commenced with Salvinia natans. A set of ten dishes was 
prepared, divided into two series of five dishes each. Those of Series I, 
numbered from 1 to 5, each contained 250 c.c. of Detmer’s solution, and 
those of Series II, numbered from 6 to 10, contained a similar quantity of 
this solution with the addition of the extract from one gramme of bacterized 
peat per 1,000 c.c. Into each of the dishes were put small sprigs of Salvinia 
natans. It was a little more difficult than in the case of Leimia to select 
absolutely uniform samples of this plant, but the portions in the ten dishes 
were chosen as equally as possible, each dish receiving two sprigs each 
consisting of three leaves and a bud, and two sprigs of two leaves and a bud 
each. Each dish therefore contained ten full-grown leaves and four buds. 
Four similar sets were chosen at the same time and their dry weights 
estimated. The figures obtained were 10-4, io-o, io-o, and 10*2 mg. 
respectively, giving an average of 10*15 mg. as the original dry weight of 
each set. The under surface and the roots of these plants when received 
were closely covered with a mass of blue-green algae, which was carefully 
removed by washing before the experiment commenced. The dishes were 
surrounded with black paper as explained above, and protected from dust. 
Twice weekly the liquids were renewed, and a weekly record was made of 
the number of fully-opened leaves in each dish. At the end of seven 
