CH. Ill] LEMNA. 63 



Instructions for determining P 2 O 5 in the ash are given 

 in Part II. 



In this as the preceding experiment it need scarcely 

 be pointed out that it is much better to start five or six 

 separate cultures under each set of conditions than to 

 rely on one only. If all develope well, the mean result 

 of the best three may be taken in each case. 



(77) Experiments with Lemna. 



Though as above stated water-plants are not generally 

 to be recommended, yet we have found Lemna minor 

 useful for purposes of demonstration. They grow rapidly, 

 and their increase being principally in one plane is easily 

 noticed at a glance. Moreover a rough numerical estimate 

 of the amount of increase in a given time can be made 

 by counting the fronds; thus in fig. 11 the culture S, 

 which has about 21 fronds, consisted originally of six 

 separate fronds, as shown in culture W. 



We grow the Lemna in narrow cylinders containing 300 

 c.c. of fluid ; if the cylinders are darkened by black card- 

 board covers the cultures keep reasonably free from algse. 



Fig. 11 gives the result of an experiment carried on in 

 a greenhouse in the winter. Three jars S, K, W, were 

 prepared, in each of which six fronds were placed. S 

 contained 0'25 /o Sachs' mixture of salts; K contained 

 0'25 % potassium nitrate, while W contained only distilled 

 water, a drop of dialysed iron being added to each 

 culture. The amount of increase is shown in the figure, 

 the difference in root production as well as in the amount 

 of frond is noticeable. In this and similar experiments 



