CHAPTER V 

 ADJUSTMENT TO TEMPERATURE 



III, Relations of plants to temperature. In its action upon 

 plants, temperature is like water, but unlike light in that it has 

 more or less to do with nearly every function. It is not only a 

 necessary condition for the chemical changes that occur every- 

 where in the protoplasm, but it also furnishes the energy for some 

 of them. As a stimulus it differs from both water and light, since 

 the responses to it are not localized in a particular organ, but are 

 found throughout the living tissues. It has, moreover, no forma- 

 tive effect upon the plant, i.e., it cannot change its form or struc- 

 ture, except in so far as growth has a bearing upon these. As in 

 the case of all factors, habitat plays a very important part in deter- 

 mining the influence of temperature upon each species. The 

 latter has been accustomed for countless generations to certain 

 extremes of heat and cold, as well as to certain seasonal sums. 

 Temperatures beyond these extremes check the plant's activity, 

 and the same is true of a deficiency in the total heat available 

 during the growing period. The effect of habit is well shown 

 by many seeds and spores, which are merely plantlets securely 

 protected against drouth. These have accommodated themselves 

 to a long period of cold, in consequence of which it is often impos- 

 sible to cause them to germinate without subjecting them to 

 cold, either naturally or artificially. 



Temperature is directly concerned, either as a necessary con- 

 dition or as energy, with every function of protoplasm. In a large 

 measure, moreover, it controls the movement of the protoplasm. 

 It is especially requisite in all the changes that have to do with 

 the assimilation of elaborated food, and in a general way, at least, 

 may be said to control growth. Under certain conditions, plants 

 show changes of position which are caused by heat and may be 



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