Went — 155 — Thermoperiodicity 



26°, and the optimal nyctotemperature dropped from 30° in young plants 

 to 8° for full grown plants; the same gradual decrease in optimal nycto- 

 temperatures was found in blossoming, fruit set and fruit development. 



In botanical literature practically no references are found to thermo- 

 periodicity. ScHiMPER (1898) refers to observations of a peach grower, 

 showing that optimal development of peach fruits requires a gradual rise in 

 temperature from blossoming to fruit ripening, and a daily temperature drop 

 from day to night, with an amplitude of about 3-5 °C. 



Bonner (1943) presented data, from which it can be concluded that 

 Cosmos grows to about twice the weight at an 18° nyctotemperature and 26° 

 phototemperature, when compared with constant 18° or 26° C. temperatures. 



Not only growth but also other processes such as rubber formation in 

 guayule (Parthenium argentatum) are strongly thermoperiodic (Bonner 

 1944). Rubber is formed at the fastest rate at nyctotemperatures between 

 5 and 10° C, and is very slight above 15° C. But rubber formation only 

 occurs provided the phototemperatures are fairly high (18-26°C.). 



Many plants do not grow or even die when the nyctotemperature is 26° 

 or higher. Lewis and Went (1945) showed that Baeria chrysostoma and 

 various other California spring annuals do not germinate, and young or older 

 plants die when subjected to 26° nyctotemperatures. This is not due to dis- 

 eases or pests, for Loo (1946) showed that even under sterile conditions 

 Baeria plants die at such high nyctotemperatures. Since the lethal effect of 

 high nyctotemperatures can be counteracted by lengthening the photoperiod, 

 it seems possible that excessive respiration, coupled with deficient carbohy- 

 drate supply of the growing regions, is responsible for death. 



Roberts (1943) in a short note reported on experiments with a wide 

 variety of plants grown at either 24 or 13°C., at day or at night. He con- 

 cluded that "The temperature during the dark period of the day is an im- 

 portant factor affecting bloom induction as well as some other reactions." 



In commercial greenhouse culture it is well known that during day and 

 night the temperatures should be kept at different levels. In Laurie and 

 KiPLiNGER (1944) the following optimal temperatures in degree Centigrade 

 are given : 



Day Night 



Violet 8.5-14 4.5-10 



Snapdragon 14 -16 7 -9 



Lathyrus 13 -15.5 9 -10 



Roses 21 -23 14.5-16.5 



Orchids : — 



Seedlings in general 21 -29 



Seedlings of Odontoglossum 13.5-15.5 



Mature plants of Cattleya lS.S-18.5 



Mature plants of Odontoglossum 10 



In many instances only optimal nyctotemperatures are given, apparently 

 because they are considered more important, and since day temperatures 

 are so hard to control. In some cases special mention is made of difference 

 in optimal phototemperature according to light intensity (Lathyrus 13° on 

 cloudy, 15.5° on sunny days) ; in another instance a differentiation of the 



