2 20 RATE OF GROWTH 



for respiration (see previous chapter), (v.) Appropriate food 

 material is essential. The plant must have some food-bodies 

 which it can convert into protoplasm and other substances 

 constituting the body of the growing parts. This does not 

 imply that the food-bodies must come from outside the grow- 

 ing plant. They may be already there, stored up as reserve 

 material. Growth does not therefore necessarily imply an 

 increase in the amount of solid substance composing the plant. 

 Indeed, we have learnt that there is no increase in the dry 

 weight of seedlings grown in darkness and supplied with only 

 pure water and oxygen. (vi.) The temperature must be 

 suitable. A plant placed in too hot or too cold a place 

 will not grow. 



GROWTH IN LENGTH. 



It is more convenient and instructive to study growth in 

 length of stems and roots than to follow their growth in thick- 

 ness. 



CIRCUMSTANCES INFLUENCING THE RATE OF GROWTH 

 IN LENGTH OF STEMS AND ROOTS. 



Temperature. — If we keep a plant in a very cold place its 

 stem will not elongate ; if we gradually raise the temperature, 

 a point is reached at which the plant is just warm enough for 

 the stem to commence to elongate. This is the minimum (or 

 lowest) temperature for growth in length of the stem of that 

 particular plant. Warming the plant still further, the stem 

 elongates faster and faster as we raise the temperature up to 

 a certain point. At this particular temperature the growth in 

 length is most rapid ; it is the best, or the optimum, temperature 

 for growth in length (for that particular plant). If we still con- 

 tinue to place the plant in warmer and warmer places, every 

 rise in temperature above the optimum causes growth to be- 

 come slower and slower. We are, so to speak, overheating 

 the plant. Finally, a temperature is reached which is the 

 highest at which the stem can elongate — this is the maximum 

 temperature. Placing the plant in a place the temperature of 

 which is higher than the maximum, the stem does not elongate. 



Effect of water supply. — If we do not supply water to a 

 plant, growth in length of stem becomes gradually slower and 

 slower till it ceases. 



