FUMIGATION OF CITRUS PLANTS. 39 
reason for the less amount of injury in daylight fumigation with 
liquid hydrocyanic acid. This is shown by Table III, in which are 
presented data giving the increase in temperature at different parts 
of a 12-foot tree covered with a canvas tent, December 17, 1919. 
Taste III.—Rise in temperature at different points within a tented citrus tree in the 
sunshine at varying periods following the covering. Thermometers placed from 6 to 16 
inches from the canvas. 
Rise in temperature. 
Minutes 
afterstart.| Sunward | Sunward Shade 
side, 11 feet | side, 4 feet | side, 4 feet 
altitude. altitude. altitude. 
5 8 3 2, 
10 16 9 3 
15 Ape WW 5 
20 26 12 6 
a@ Sun temperature at start, 75° F.; shade temperature, 69° F. Time, 10.43 a. m. 
It is shown in this table that an increase of 26° occurred toward 
the top of the tent on the sunward side within 20 minutes after 
covering, whereas at the same time on the same side 4 feet from the 
ground the increase was only 12° and on the shaded side of the tree 
at thesame height only 6°. Injury in daylight work has been observed 
to be proportionately greater at these different points in the case of 
pot-generated gas. The lower part of the shaded side of the tree, at 
which the temperature increase is very slight, is seldom injured even 
when very severe burning takes place on the sunward side of the tree. 
The effect of the temperature is least felt in the case of thoroughly 
hardened trees. In fact, the extent to which sunshine fumigation can 
be practiced during the winter period is attributable largely to the 
extent of this hardened or dormant condition of the trees. This 
condition also offers an explanation for the safety of fumigation at 
very low temperatures, sometimes at the freezing point, whereas at 
other times, especially in the early fall, while trees are active, severe 
injury takes place at several degrees above the freezing point. In the 
Tulare County citrus belt of California the writer has noted night 
fumigation, with heavy dosages, carried on during the summer at 
temperatures as high as 85° F. without apparent injury to the plants. 
Fumigation in the coast region of southern California at such high 
temperatures would produce severe injury. He believes that this 
greater safety in the northern citrus region is due largely to the more 
resistant condition of the plants brought about by the very hot, arid 
chmate during the summer. Duggar (4) states that green leaves ex- 
posed to sunlight show a temperature from 2° or 3° to 15° higher than 
the air, and according to MacDougal (10) the maximum temperature of 
higher plants varies from 100° to 115° F. At or above the maximum 
