Growth o/Lemna minor in Mineral Culture Solutions. 483 



their original size of 3 - 73 sq. rnm. throughout the experiment, whilst the 

 control plants soon began to show signs of starvation, and decreased in size 

 week by week, until at the end of the experiment they had diminished from 

 3 - 73 to 078 sq. mm., and it became difficult to count and measure them 

 accurately. 



Microscopic investigation also showed striking differences in internal 

 structure. In the control plants there was an excessive number of large 

 air spaces. The individual cells were small, with a large central vacuole 

 and a small nucleus. In the auximone plants the tissues were more 

 compact, with fewer and smaller air spaces. The large cells were filled with 

 cytoplasm and possessed a well-developed nucleus which stained deeply. 



In these experiments there was only one dish of plants for each variation 

 of culture solution, and it was considered that " the result could not be 

 regarded as conclusive, owing to the probability of experimental errors." It 

 was therefore necessary to repeat and extend the experiments with a larger 

 number of dishes for each culture solution. After consultation with certain 

 botanical colleagues, it was decided that each set should consist of 10 dishes, 

 that the rate of growth should be estimated by (1) increase in number of 

 plants, (2) increase of dry weight, instead of measurement, each time the sets 

 were halved, and that other extracts of bacterised peat should be tested. 



Owing to the lack of suitable greenhouse accommodation at King's 

 College for these experiments, the authorities of the Botanical Department 

 of the Imperial College of Science and Technology, South Kensington, 

 kindly granted permission and offered facilities for the work to be carried 

 out in the greenhouse laboratory of that college. 



Experiments with Ordinary Distilled W ater. 



On June 9 last year, five series, each consisting of 10 dishes, were 

 prepared, and were numbered from 1 to 50. Flat-bottomed glass crystal- 

 lising dishes of 4 inches diameter were employed, all containing 250 c.c. of 

 the required solution. Each of the dishes was then enveloped on the 

 outside, to the level of the contained liquid, with paper which was dull 

 black on one side and white on the other, the black side being towards the 

 dish and the white towards the exterior, in order to prevent as much as 

 possible access of any heat and light rays through the sides and bottom of 

 the vessels. 



The solutions employed were : — Series I, Detmer's standard culture 

 solution ; Series II, Detmer's solution, together with a water extract of 

 bacterised peat ; Series III, Detmer's solution, with a similar extract freed 

 from humic acid ; Series IV, Detmer's solution, plus an alcoholic extract of 



2 T 2 



