Vernalization and Photoperiodism —114— A Symposium 



has probably been driven northward, rather than southward into equatorial 

 regions, to find optimum conditions of survival. 



Development of the Climbing Habit: — A large number of her- 

 baceous and semi-herbaceous plants in both the tropics and in the warm 

 temperate regions have become climbing forms. The suggestion has been 

 made that these are herbaceous types because of the adoption of the vigor- 

 ous climbing habit (Sinnott and Bailey, 1914, p. 594) but this concept 

 appears to have little to recommend it as an explanation. The writer's 

 studies of the length-of-day responses of a great variety of herbaceous 

 plants indicate that the assumption of the dwarf bushy or the climbing 

 habit in many instances is a direct response to particular lengths of day. 

 Some plants have been induced to become climbers in response to short 

 days, and others have shown the climbing behavior only when subjected to 

 long days. Instances of these responses have been shown by members of 

 different genera of the Leguminosae, including Canavalia, Phaseolus vul- 

 garis, P. lunatus, P. coccineus, and the wild woodland bean, P. polystachios 

 of eastern North America. Differences in the climbing response have been 

 especially clearly shown in the case of the last named species, studies of 

 which were reported by the writer (1938). This bean developed a low 

 bushy habit of growth in response to constant lengths of day of 10, 12, 

 IZyi and 13 hours. The climbing habit was not shown until lengths of day 

 of 13 1/2 hours or longer were experienced. 



It is obvious, then, that if some great Tertiary transformation of cli- 

 mate had occurred that brought about marked changes in seasonal length 

 of day giving the herbs an ascendency over all the cooler regions of the 

 earth, and this event was followed by extensive migrations into all other 

 favorable regions, the viney habit in many instances would also be called into 

 response or in some instances repressed by this selective force alone of the 

 climatic complex. 



Length-of-Day Responses of the Woody Angiosperms : — Little 



mention has been made of the length-of-day responses of the woody plants 

 in the present discussion. The annual herbaceous plants have usually been 

 subjected to study by investigators because of the greater ease with which 

 these in a short time can be grown to the flowering stage. The writer, 

 however, in various publications has shown that the flowering of many of 

 the woody plants is quite as responsive to length-of-day as the flowering 

 of the herbaceous plants. This has been found true for a number of tropi- 

 cal and subtropical species, including Poinsettia, Euphorbia pukherrima, 

 Bougainvillea, Bougainvillea glabra and others. These two warmth-de- 

 manding plants are strictly short-day types, being able to flower only when 

 the day length does not exceed 12 to 12>4 hours. Such plants are of 

 necessity confined to tropical regions not only because they find favorable 

 warm temperatures here but because short days also constitute a feature 

 of the climatic complex and make their existence possible. 



Another tropical woody plant known as the Turkscap Hibiscus, Mal- 

 vaviscus conzattii, is day-neutral in its behavior and flowers readily in re- 

 sponse to long days (14i4 to 15 hours) as well as to short days of 10 hours 

 duration. 



