ate ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
plants as a result of the application of aluminum sulphate is all the 
more remarkable because the soil in which the recovery took place 
had been in the pots for more than a year, subjected to the leaching 
action of the customary greenhouse watering. 
That the aluminum sulphate had no direct fertilizing effect is evi- 
denced by an earlier experiment, in which, when it was added to a 
soil composed of peat and sand, rhododendron seedlings showed no 
greater growth than in untreated peat and sand. The aluminum 
sulphate contributed no beneficial effect through the development of 
an acid condition, because the soil was acid already. If the aluminum 
sulphate had contributed plant food the rhododendrons would have 
made increased growth. 
The plants concerned in this resuscitation experiment were re- 
potted in the following year and the treatment of the two lots was 
continued as before. The plants to which aluminum sulphate had 
been applied remained healthy and reached the height of about 5 
inches. All the others died. 
The effect of aluminum-sulphate treatment when carried through 
a third year on rhododendron seedlings is illustrated in Plates 
5 and 6. The plants in Plate 5 were from a lot that were first 
potted in 2-inch porous earthenware pots on May 3, 1921, in the 
standard neutral soil already described, equal parts of loam, manure, 
and sand. They remained in this injurious soil throughout the 
years 1921 and 1922 without repotting. Many of them died. On 
March 23, 1923, 21 of these plants, which were still alive, were 
repotted in 38-inch pots in the same neutral-soil mixture. Plate 5, 
from a photograph taken February 12, 1924, shows that some of 
these plants were dead and all the rest were sick. Plate 6 shows 
a lot of plants identical in origin, age, and treatment with those in 
Plate 5, except that they had received two applications of aluminum 
sulphate. The details are as follows: On May 3, 1921, after potting 
in 2-inch pots in a soil consisting of equal parts of loam, manure, 
and sand, each plant was given one-third gram of aluminum sul- 
phate dissolved in water. They were not repotted in 1922, and not 
having been chilled in the winter of 1921-22 they made no new 
growth in the following summer.* On January 16, 1923, they were 
moved from the warmhouse (55° to 70° F.) to the coldhouse (35° 
to 40° F.) for chilling, and on March 23, 19238, they were repotted 
in 8-inch pots in a mixture of equal parts of loam, manure, and 
sand, to which one-half of 1 per cent, by bulk, of ground aluminum 
sulphate had been added. The plants grew vigorously during the 
season of 1923. On February 12, 1924, when the photograph used 
8 The trees and shrubs of cold climates, after becoming dormant, do not start into 
growth again in a normal manner until they have been subjected to a period of chilling. 
See “ The influence of cold in stimulating the growth of plants,” Smithsonian Report for 
1919, pp.'281 to 291, pls. 1 to 27. 
