100 
GARDENING IN THE SOUTHWEST 
Factors 
in Rate 
of Plant 
Growth 
is light, and the process of food-making taking place in the green 
leaves is called photosynthesis, "Building up by Light”. 
The rate of plant growth is limited not only by the peculiarities 
of the plant itself but also by external factors, such as light inten¬ 
sity and duration, carbon dioxide supply, temperature, and water 
supply. In nature a plant rarely has an opportunity to grow as its 
maximum rate. Possibly many would grow a little faster if the days 
were a little moister, or warmer, or the sun a little brighter. In a 
greenhouse these conditions can be varied and are easily controlled, 
so scientists have long been able to tell us what are the highest and 
the lowest temperatures that a given plant will endure, and also 
what temperature leads to the greatest development. 
One of the hardest things to study is the carbon dioxide supply, 
but recent apparatus, which permits the investigator to control the 
supply of that gas makes it apparent that it too is often the factor 
that limits the rate of the amount of growth. Three-hundredths of 
a per cent of carbon dioxide gas is present in average air. If in 
the course of an experiment the carbon dioxide is kept at ten times 
the average amount or three-tenths of a per cent, and the other 
growth factors are kept at a favorable point, plants kept in these 
conditions show a great increase in rate of growth. If the plants 
are given not only more CO 2 , but also a longer period of light, that 
is, if the normal day is supplanted by artificial light, the increased 
rate of growth is startling. 
A room at Boyce-Thompson Institute, the winter of 1928, was 
equipped with powerful lights, so powerful that their light closely 
approached sunlight, both in intensity and quality. Temperature, 
humidity, and carbon dioxide supply were closely controlled. Beside 
this room was another exactly like it, except it was perfectly dark. 
A variety of plants were placed in these rooms, some of them stay¬ 
ing in the light all the time and some being moved on a regular 
schedule between the light and dark rooms. By means of this 
experiment it has been determined how the length of the day in¬ 
fluences plant growth. Of course, these plants had only artificial 
light, but others like them were kept in greenhouses with normal 
daylight and in many cases grew much more slowly than the plants 
in the constant condition room. Plants grow at an astonishing 
