119 
Birner (1881) experimented on the amount of water needed by 
potatoes. Jour series of experiments were made, each including five 
tubs having different amounts of water, as shown in the following 
table, which gives the average of the four series: 
Harvest 
weight of 
ubers. 
Bee 
| Aver- 
| (plant. “fiber. 
P.ct. |\Grans. Grannis.) 
1 |80-60 | 809 | 42 | 
2 | 60-40 628 46 | 
3 | 40-30 413 42 | 
4 [ered 313 | 34 | 
5 | 20-10 214 23 | 
| | | 
These figures show a steady increase in the amount of harvest with 
increasing moisture. 
The student will notice that in these experiments where the plants 
are kept in tubs under protection from natural rains the watering and 
growth go on under continued sunshine. The experiments therefore 
correspond with the case of irrigation in a dry, sunny climate, and it 
is not to be understood that the same amount of water deposited 
naturally by clouds, with attendant long-continued obstruction of 
sunlight and heat by the clouds, would have produced the same large 
crops. 
R. Heinrich (1876) experimented at Mecklenburg on the influence 
of water on grasses and clover. Ten sets of tubs filled with sterile 
sand were sown with grasses and clover and watered daily, with 
results as shown in the following table: 
. ._ | Harvest || - | Harvest 
Weight  PGtv- | weight | Weight | EGuiv- | Vveight 
Tub.| of daily | anil |offresh- }|Tub. of daily anil, of fresh- 
water, See | cut water. raineallel cut 
‘| grass. | ‘| grass. 
a : a Eee 
Grams. mm. Grams. Grams. | mm. Grams. ~ 
1 100 | 1 35 || 6 600 | 6 138 
2 200 2 44 || 7 | 700 7 148 
3 300 3 sy || 8 | 800 8 161 
4 400 | 4 8 || 9 | 900 9 156 
5 500 | 5 110 || 10 1,000 10 170 
| 1] 
This shows that the harvest increased steadily up to 8 millimeters 
of rainfall daily, but for 9 or 10 millimeters per day the increase in 
harvest was so slight that we may consider 9 millimeters, with an 
average harvest of 162 grams, as the best that could be obtained under 
the temperature and sunshine prevailing that year at Mecklenburg. 
Doubtless a different quantity of water would be required in order to 
obtain the maximum harvest in other climates. 
