40 THE FLOWERING PROCESS 



There is often a very close relationship between response to day- 

 length and latitudinal location of a given plant. H. A. Mooney (66), 

 a student of the ecologist Dwight BilUngs at Duke University, has 

 studied the day-length requirements of a small alpine plant, Oxyria 

 digyna which grows not only in the alpine tundra of mountain ranges 

 in North America, but also in the Arctic. He found that samples 

 collected from various latitudes throughout the Rocky Mountains 

 and the Arctic each responded in a specific way, depending on the 

 location from which it was collected. As collections were made 

 farther and farther north, longer days were required to induce 

 flowering. Even though all of his samples belong to the same 

 taxonomic species, they were genetically different and very accurately 

 adapted to the region in which they were found. 



In recent years, other studies of this type have been made with 

 increasing frequency. Workers at the North Carolina Experiment 

 Station (62), for example, have studied more than 30 species and 

 varieties of cotton {Gossypium) under a combination of day-lengths 

 and temperatures. They found a wide diversity within the genus. 

 Most were favored by short days and cool nights, but at least one 

 species clearly initiated flowers more readily under long-day 

 conditions. 



Perhaps the most detailed work of this type with a single genus 

 has been done by Bruce G. Gumming (47), in the Ganada Department 

 of Agriculture at Ottawa. For a number of years now he has been 

 studying the responses of pigweed {Chenop odium) to environmental 

 conditions, his work being at least partially directed towards fijiding 

 better ways to control these weeds in agriculture. He has determined 

 the sensitivity of 33 species to day-length, for example. He found 25 

 short-day plants and 8 long-day plants (at least in a quantitative 

 sense). With some of these, germination, response to nutrients, 

 chromosome numbers, lengths of shoots, primary stems, and petioles, 

 and number and sizes of leaves as well as flowering have all been 

 studied in various environments. In the course of this work Gumming 

 discovered the special variety of Chenopodium rubrum which will 

 respond to a single dark period and flower in a petri dish as a seedUng. 

 Figure 3-8 shows the flowering response of six varieties of Cheno- 

 podium album, collected from diff'erent latitudes in North America. 

 Some (varieties E, D, and F) are quite clearly short-day plants, but 



