CHAPTER XXVII 

 SPECIFIC GROWTH FACTORS 



When ye hae naething else to do, ye may be . . . sticking in a 

 tree; it will be growing, Jock, when ye're sleeping. 



— Scott. 



The Growth Effect. — A factor which influences growth may do 

 so in one of three ways. (1) The direction of growth may be 

 changed so that movement results as when a plant bends towards 

 the light (Chap. XXIX). (2) A factor may influence the amount 

 of growth and (3) it may also affect the nature of the organs produced. 

 This latter is commonly called the morphogenic effect. The direc- 

 tion of growth will be discussed under " movement," but the effect 

 of external factors upon the amount of growth and the organs 

 produced will now be treated in some detail. 



Heat. — For growth a minimum temperature is necessary. For 

 most plants 5° C. is the minimum, 30° the optimum, and 45° 

 the maximum. Opuntia will grow at 58° C. (MacDougal), which 

 is about the high temperature record for growth among seed 

 plants. As samples of various plants the following figures are 

 included, which show that the optimum is much nearer the max- 

 imum than the minimum: 



Plant 



Wheat 

 Peas 

 Corn 

 Melons 



Wheat, as these figures show, can be grown at a much higher 

 latitude than corn and melons, which require rather high mini- 

 mum temperatures. There is a great variation in plants in this 

 respect. In tropical plants 10-15° is a minimum, in the tem- 

 perate zone 5-10° is generally the minimum, while in the arctic 

 regions plants grow well at a temperature of 5-10°, the minimum 

 being close to the freezing point. At the other extreme are the 

 algae found in hot springs, where they thrive at 80° C. and fail to 

 reproduce if the temperature falls below 40°. 



When dry, plants can endure much greater extremes than when 



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