GRO WTH 
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, 
some external, some internal. Light retards growth, as is 
proved by measurement of the rate of growth of the same 
plant during day and night, and as is also evidenced by the 
phenomena of etiolation (growth of green plants in darkness ; 
under these circumstances the shoots grow rapidly, but are 
very slender, the leaves small and far apart;; chlorophyll is 
not developed, but a yellow matter termed etiolin is). Below a 
certain temperature (varying according to the plant and the 
climate) growth will not take place. As the temperature 
rises from this minimum, the rate of growth increases, till it 
reaches its maximum at a temperature which is termed the 
opti?num temperature for growth in that plant ; as the tem- 
perature rises beyond this, growth decreases, till a maximum 
temperature is reached, beyond which growth will no longer 
go on 1 . The rate of growth is also affected by conditions 
internal to the plant, especially the amount of food supply 
and the rapidity with which it is made available. The 
growth of seedlings or of buds opening in spring is usually 
more rapid than that of parts which have not the advantage 
of a rush of food from some reserve store, but depend upon 
that provided from day to day by the leaves. 
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 Most plant functions show similar tonic effects of temperature. 
3—2 
