GROWTH 
35 
Growth takes place only under suitable conditions, viz. 
(i) a supply of material to the growing parts, (2) a supply 
of energy, (3) an adequate temperature, and (4), in most 
cases, a supply of sufficient water to the growing parts 
to maintain the cells in a condition of great turgidity or 
tension. The supply of material is brought from the leaves 
or from storage places; the energy is obtained from this; 
that an adequate temperature is necessary is a familiar 
annual experience in temperate countries: and that turgidity 
is needful is easily proved by simple experiments, and 
indicated by the fact that a plant will not grow unless well 
supplied with water. 
The rapidity of growth depends on various factors, 
external and internal. It used to be thought, because a 
plant grew faster at night, that light had a strong retarding 
influence upon growth, but Lock, working at Peradeniya 
with giant bamboos, showed that light had no effect upon 
the growth, which closely followed the humidity of the air. 
This, with his own work on assimilation, &c., suggested to 
Blackman the true explanation, which has been borne out 
by experiments at Peradeniya by Smith 1 . Humidity, food- 
supply, temperature, &c., have all an influence upon growth, 
which increases as they increase, and is limited only, up to 
the maximum at which it stops, by defects in these factors, 
which should all increase together to give the maximum 
growth. The factor which first runs short is called the 
limiting factor. Thus in the growth of giant bamboos at 
Peradeniya, the temperature is high enough, but the water 
supply is limiting, while at Hakgala, a colder place, it is the 
temperature which is limiting during the night, the air being 
then saturated. When moisture is limiting, no increase of 
food will increase the growth, and so for other factors. 
Other functions seem to show similar results. 
The direction of growth, unless influenced by greater 
stimuli on one side of the organ, is usually straight forward, 
but the tip commonly nutates , i.e. moves alternately from 
side to side or round an elliptical or circular orbit, owing to 
varying rate of growth of the tissues : as the growth becomes 
1 Lock, Ann. Perad. II. 211, 1904; Blackman, Optima &>c.y Ann. 
Bot. xix. 281, 1905; Smith, Ann. Perad. ill. 303, 1906. 
3—2 
