672 



PAET IV. THE PHYSIOLOGY OF PLANTS. 



It may be stated generally that all the functions of plants 

 inhabiting temperate climates begin to be carried on at a tempera- 

 ture a few degrees above the freezing-point ; as the temperature 

 rises to 25°-30° C. the activitj'- of the functions is increased and 

 the optimum attained ; with a further rise the activity of the 

 functions is diminished, and at 45°-50° C. they commonly cease 

 altogether. In the case of plants which naturally grow in warmer 

 climates, the minimum-temperature is somewhat higher than that 

 stated above. Thus a pumpkin-seed will not germinate at a 

 temperature below 13° C. 



The power of withstanding the injurious effect of exposure to too 

 high a temperature depends mainly upon the proportion of water 

 which the plant, or any particular part of it, contains. Thus, dry 

 peas can withstand exposure for an hour to any temperature up to 

 70° C, whereas, when they have been soaked in water, exposure 

 to a temperature of 54° C, proves fatal. Most parts of plants are 

 killed by prolonged exposure to a temperature, in air, of about 

 50° C, and in water, of about 45° C. 



Injury or death by exposure to cold, is only induced when the 

 temperature falls — in some cases many degrees — below freezing- 

 point. Some plants — just those, namely, such as Lichens, and 

 some Fungi and Mosses, which can undergo desiccation without 



injury — are not killed by exposure 

 to low temperature. Here, also, the 

 liability to injury depends upon the 

 amount of water contained in the 

 tissue. Thus, dry seeds and the 

 winter-buds of trees can readily 

 withstand low temperatures ; but 

 when they contain a considerable 

 quantity of water, as when the seeds 

 are germinating or the buds unfold- 

 ing, they are very susceptible to 

 injury. When a part of a plant, 

 which contains a large proportion 

 of water, is exposed to a low tem- 

 perature, a portion of the water 

 contained in the cells escapes from 

 them and becomes frozen on their 

 surface, the whole tissue at the same time contracting ; the water 

 does not freeze in the interior of the cells. The water which has 



Fig. 467.— Transverse section of a 

 frozen leaf-stalk of Cynara Scolymus .- 

 e the detached epidermis ; g the paren- 

 chyma in which lie the transverse 

 sections of the vascular bundles (left 

 white) ; K K the incrustation of ice 

 consisting of densely-crowded prisms 

 (the cavities of the ruptured tissue 

 are left black in the figure). 



