108 



THE AMERICAN BOTANIST 



Periodicity in Plants. — Rarely, if ever, does growth 

 proceed continuously, even in the tropics. In manv northern 

 regions, the winter puts a stop to growth, and a drouth in 

 summer may do likewise, but in the tropics where warmth and 

 moisture are always plentiful the plants still have their resting 

 spells and often take a vacation at what seems the most in- 

 opportune of times. Resting is also more of an individual 

 peculiarity in the tropics and one plant may suddenly shed its 

 leaves when the other members of its species retain them. Xot 

 infrequently, a single branch may thus rest, or it may resume 

 growth as other parts of the plants are preparing for a dormant 

 period. Much evidence has been accumulated to show that 

 periodicity in plants is due to internal causes, but the prevailing 

 idea seems to be that it is due to external conditions which are 

 determined largeh' by heat, moisture and oxygen. It would 

 be difficult, however, to explain some cases of periodicity by 

 these latter factors. For instance, trees rapidly increase in 

 growth after midsummer, regardless of either temperature or 

 moisture. In July or August, new rifts in the bark of all our 

 common trees show how rapidly new wood and bark are being 

 added to the trunks. 



Canada Lily in Northern Iowa. — The meadow lilies 

 in the writer's locality (Emmet County, Iowa) are found 

 rather sparingly on low prairies but are abundant in a local 

 meadow where they present quite a display when in bloom. 

 The flowers of these plants differ somewhat from the typical 

 Lilium Canadcnse of the manuals. They are orange or orange- 

 red and have the perianth segments not merely spreading as 

 in the typical form, or even turned back as in the Turk's cap 

 lily (L. siiperhuin) but they are reflexed and closely curled 

 together, meeting and overlapping behind the flower. Thus in 

 a fully opened flower, not even the points of the perianth 

 divisions project outward or backward but are curved together 

 so that both the back and face of the flower present somewhat 



