[Vol. 2 



300 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



least increased seasonal vigor in certain elements of the flora 

 is found in all zones, a phenomenon of mixed dependence upon 

 light and temperature. It is most marked in the Temperate 

 zones but is to be found in the Subtropical, Lower Boreal and 

 Lower Austral zones as well. In the Upper Boreal and Upper 

 Austral zones its appearance is perhaps more associated with 

 varying intensity of light than with temperature, and it is 



least pronounced in the Tropical zone, where it seems to be 

 wholly dependent upon light variation. 



It is certain that many boreal summer species appear as 

 winter or early spring species in the Temperate zone and like- 

 wise certain temperate species appear during the colder sea- 

 son in the Subtropical zone. There is some, but apparently 

 not very much, overlapping between the upper portions of 

 the Subtropical zones and the Tropical zone. From the very 

 incomplete studies thus far made, it seems that most species 

 range through from 5 to 10° C. of temperature, that each 

 zone has its own characteristic species and that extensions 

 up to 15° C. for active growth and reproduction are few, if 

 at all existent. More careful examination, however, is neces- 

 sary to satisfactorily demonstrate this last point. 



While the limits of the temperature zones have been founded 

 on the isotheres or lines of average daily summer temperature, 

 seasonal phenomena cause us to consider also the isocrymes 

 or lines of average daily winter temperature, especially as to 

 overlapping or transitions between the zones. The isocrymes 

 are of especial importance in those portions of certain zones 

 where, especially on account of strong currents, the seasonal 

 variation is extreme, e.g., on the eastern coast of North 

 America and on the eastern coast of Asia. In such regions 

 there may be expected extreme expression of seasonal change 

 of flora. 



The disturbances of distribution due to upwellings cause 

 confusion in the tabulated results unless they are to be defi- 

 nitely accounted for. This confusion is greatest at present in 

 connection with the species of the central coast of California. 

 Spot distributions also cause the species concerned to be tabu- 

 lated in more than one, or, if combined with seasonal disturb- 



