Chap. II] 



HEAT 



45 



The corresponding cardinal points, particularly the optima of the 

 individual functions, differ from one another only by a few degrees or 

 fractions of a degree in the case of plants of an equable climate, whereas 

 they may differ greatly from one another in regions with extreme 

 temperatures. Indeed, it may happen in such climates that the curves of 

 temperature of definite functions do not touch those of others. Long ago 

 the practical experienced man grasped these facts, which were ignored by 

 phaenologists, and now he rears tropical plants at a uniformly high 

 temperature, but temperate plants at temperatures alternately high 

 and low. 



The oecological optimum temperature does not remain constant during 

 the whole development of a plant — at least in temperate regions — but, as 

 Sachs' investigations prove, shows a rise as development proceeds, so that, 

 for instance, the temperature which is most favourable for the processes of 

 germination does not reach the optimum for succeeding functions. We learn 

 too from the art of fruit-forcing that we must regard the rise not as constant, 

 but as oscillating. Though the harmonic optimum temperature for the 

 successive stages of development is alternately higher and lower, the main 

 curve nevertheless shows a marked rise. Very instructive in this connexion 

 is the following table, prepared by the horticulturist Pynaert, showing the 

 most favourable temperatures (oecological optima) in forcing the peach-tree : 



OECOLOGICAL OPTIMA 



IN THE FORCING OF THE PEACH-TREE 

 (after Pynaert). 



Up to the present time only a few satisfactory investigations have been 

 made to fix the cardinal points of the individual functions. The most 

 thorough-going of the available experiments concern germination, a process 

 made up of various distinct processes, such as swelling, the actions of 

 ferments, transport of plastic material, generation of energy, cell-division, 

 cell-growth, &c, each of which possesses its own cardinal points. The data 

 which have been fixed in reference to germination are therefore not very 

 valuable from a purely physiological point of view, but this complexity 

 does not impair their value in relation to oecology and geographical botany, 



