iy 
3 
| 
StS any Seep a eS 
i eh eee ll 
a 
1903] FLORA OF NORTH CAROLINA 249 
as physiologically adapted to the influences of a previous short 
glacialsummer. This rapid growth is insured by the activities 
of the plant during the previous short season in storing up large 
amounts of reserve material for the next brief season’s growth. 
Even under favorable conditions of summer heat, every external 
perceptible vital motion in these boreal plants ceases, and it is 
only after a dormant period of some months, that growth can 
commence anew. This periodic alternation of vegetative activity 
and rest is in general so regulated that for a given species of 
plant both occur at definite times of the year, leading to the 
inference that the periodicity only depends upon the alternation 
of the seasons, and, therefore, chiefly upon that of temperature 
and moisture. The plants of Scandinavian affinity agree physio- 
logically in having a periodic growth and rest. The following 
diagrams will illustrate this. Diagram 8B, if compared with 
diagram A, shows that the period of vegetative activity of our 
spring plants of boreal origin corresponds with an arctic or 
glacial summer, while the dormant period corresponds with an 
arctic winter, although our present summer has encroached on 
the former glacial winter. 
A. ASTRONOMICAL YEAR DURING GLACIAL PERIOD. 
: FLOWERING 
VEGETATIVE DoRMANT PERIOD 
REPRODUCTIVE 
PERIOD 
GROWING PERIOD OF PLANTS 
8 i = 
ARCTIC SUMMER ARCTIC WINTER 
B. ASTRONOMICAL YEAR — 1903. 
VEGETATION FLOWERS DORMANT PERIOD 
| 
GROWING PERIOD Dormancy | DORMANCY 
F OF |= OF 
BOREAL PLANTS SUMMER | WINTER 
——, ~) J 
PRESENT SUMMER PRESENT WINTER 
