EFFECT OF LIGHT ON VEGETATION 
101 
rate when they are supplied with everything they can use in grow¬ 
ing. Of course, artificial climate is costly and is only to be used 
for experimental purposes, but it is intensely interesting and useful 
to know the part played by everything that contributes to plant 
growth. 
One thing comes out clearly, in many plants it is the length 
of time that they are illuminated each day and not the tem¬ 
perature that brings on flowering. There are short-day plants, long- 
day plants and indifferent plants. Our Fall-blooming flowers, Sal¬ 
via, Cosmos, Dahlia, Chrysanthemum, and possibly Sunflower, are 
some of the many plants that flower on the short day. Salvia 
blooms, as a tiny seedling, in the cold frame in the Spring, but 
stops and forms no buds at all as the days grow longer. With more 
than twelve hours of sunlight the plants grow larger and larger, 
but form no buds until the days grow shorter in the Fall. When a 
twelve-hour, or shorter day arrives, bud formation begins and flow¬ 
ering continues until frost. This is not a temperature response, be¬ 
cause when conditions are controlled in the laboratory the tem¬ 
perature may be maintained at any point, and if the illumination 
is longer than twelve hours out of the twenty-four, no buds are 
formed. 
There are very few long-day plants among the greenhouse plants. 
They have no flower on a short day to do well in usual greenhouse 
conditions. So, any plants that are normally grown in a greenhouse 
are either short-day plants or plants that are indifferent. 
Influence 
of Day 
Length 
