AUard —113— Length of Day in the Past 



assemblage in southern Chile represents a subsequent extension of range 

 from this tropical group. 



There is another characteristic of plants, which may be very intimately 

 related to length of day, determining very largely their success in a given 

 climate, namely, competitive ability. Long study by the writer dealing with 

 a great variety of wild and cultivated plants has shown very forcibly that 

 the competitive ability of many annual herbaceous plants depends upon a 

 favorable length of day which will insure vigorous stature and abundant 

 seed formation. In the case of many plants, more especially the short-day 

 plants, which have adapted themselves to the shortening days of late sum- 

 mer and autumn in higher latitudes, this behavior deserves particular men- 

 tion. 



If low ragweed, Ambrosia artemisiifolia, Klondyke Cosmos, Cosmos 

 sulphureus, or beggar-ticks, Bidens bipinnata, are grown from seed on short 

 days of 10 or 12 hours from the outset, the reproductive phase begins at 

 once, and extremely dwarfed plants, only two or three inches in height, 

 will result, these flowering, fruiting and quickly dying in response to the 

 short-day conditions. Such tiny plants often appear late in the season when 

 the days have naturally shortened and may occur in far southern latitudes 

 where the seasonal length of day is always much below the critical for 

 flowering for many species. It is obvious that such dwarfed short-lived 

 plants are poorly equipped to compete with rank and vigorous flora any- 

 where and more especially in those latitudes where the longest days of the 

 season are below the critical for hastened flowering. 



Such short-day plants find their greatest success in higher latitudes 

 where a pre- vegetative period is favored by lengths of day above the critical 

 for flowering. Under such conditions the plants, by their vigorous growth 

 and increased stature, not only develop greater competitive ability in their 

 native habitat, but produce enormous quantities of seed when the shorten- 

 ing days of late summer finally initiate the reproductive phase of develop- 

 ment. Such short-day plants find their optimum development, then, not in 

 those latitudes where too short-days prevail, as in the tropics, but in those 

 latitudes where the reproductive phase follows a vigorous purely vegeta- 

 tive development which has been engendered by lengths of day in excess of 

 that which initiates flowering. This type of response would manifest itself 

 very promptly in a very large class of short-day plants, should conditions 

 of climatic uniformity involving uniformity in length of day as well as 

 conditions of warm temperatures, give way to marked seasonal and zonal 

 differentiations which appear to have prevailed during Tertiary times. 

 These changes would favor the seasonal multiplication and dominance of 

 a very large class of typical short-day plants, characterized by low com- 

 petitive ability in the tropics, but physiologically fitted to attain a very 

 vigorous development in the middle latitudes, since here they would find a 

 favorable seasonal schedule first for the pre-vegetative phase, then finally 

 for the culminating reproductive phase. At the present time this is the 

 normal life history of a great host of autumnal annual and perennial herbs 

 in the higher latitudes, and this behavior represents a timely adjustment 

 to our present marked seasonal and zonal length-of-day relations. It is 

 obvious from these relations that a very large class of short-day plants 



