CHAPTER II 

 HEAT 



i. General Considerations. Heat and the structure of plants. Effects of a cold 

 climate on the form and lie of leaves. The three cardinal points. Phaenology. 

 2. Zero Points of Plant-life. i. Lower Zero Points. Power of resisting cold. Death 

 by cold often due to drought. The coldest points on the earth, ii. Upper Zero Points. 

 Power of resisting heat. Sachs' researches. Hot springs. Highest observed tem- 

 peratures in soil and air. 3. Cardinal Points of the Functions of Plants. The 

 harmonic optimum. The absolute optimum. The oecological optimum. Fluctuations 

 of the harmonic optimum during development. The oecological optima of a peach- 

 tree. Cardinal points of germination. Germination, growth, assimilation, and respira- 

 tion at low temperatures. Useful low temperatures. Effects of cold on sexual 

 reproduction. 4. Acclimatization. Transference from a warm to a cold climate and 

 the reverse. Schuebeler's and A. de Candolle's researches. H. Mayr on the acclimatiza- 

 tion of forest trees. 



i. GENERAL CONSIDERATIONS. 



In spite of the predominating part played by temperature in regard 

 to the distribution of plants, the action of temperature is not placed in 

 this book at the head of the factors determining plant-distribution, because 

 the existence of such action on vegetable organisms is less clearly recogniz- 

 able than is that of water. We can directly observe the ingress of water 

 into a plant and its egress, we can explain physiologically the effects 

 caused by these, and we can follow the transpiration-current along its 

 course ; whereas the action of heat is carried on in the molecular region of 

 the protoplasm beyond our ken, and is visible to us only in its final con- 

 sequences, such as the acceleration, retardation or complete cessation of 

 physiological processes. The oecological phenomena display similar dif- 

 ferences. Protective adaptations against a want or superfluity of water 

 are within our power of observation, those against cold and heat are 

 entirely beyond them. We can directly see whether any plant naturally 

 inhabits a dry or a moist station, but not whether it belongs to the flora 

 of a cold or warm climate. Indeed plants from hot deserts frequently 

 have a strong resemblance in habit to those of polar zones. 



Attempts have, however, recently been made to establish some distinguishing 

 points between individual plants belonging to species growing respectively in 

 high alpine or polar, and in temperate climates. Thus Lindberg writes as 



D 2 



