SEASONAL VARIATIONS IN TRANSPIRATION RATES 211 



periodicity curve, transpiration rates being regularly greater at night than in 

 the daytime. This appears to be due to complete or nearly complete stomatal 

 closure during the hours of daylight, while the stomates are, as a rule, open 

 at night. 



Seasonal Variations in Transpiration Rates. — In temperate regions 

 transpiration occurs predominantly during the warmer months of the year, 

 and especially during those periods of the warm season when the soil water 

 supply is abundant. Among the deciduous group of woody perennials, the 

 branches are defoliated during most of the autumn, all of the winter, and 

 the earlier part of the spring. Although the twigs and branches which remain 

 exposed to the atmosphere are completely encased in corky layers of bark and 

 the buds are enclosed within cutinized bud scales some water loss occurs 

 from such species even during the winter months. Young twigs lose water 

 under such conditions faster than older ones. Winter transpiration rates of 

 deciduous woody plants are always negligible in comparison with summer 

 rates. In evergreen species of either the needle-leaved or broad-leaved type 

 the transpiration rates are usually not appreciably different at most times 

 during the winter from the transpiration rate of deciduous trees at that season 

 (Weaver and IVIogensen, 19 19). This is probably due principally to the fact 

 that the low temperatures of the winter prevent opening of the stomates 

 although the relatively low leaf water contents characteristic of this season 

 may also be a factor in maintaining the stomates in a closed condition. 



Mild periods of any considerable duration often have a detrimental effect 

 upon evergreen species during the winter months. The warm air temperatures 

 induce stomatal opening, and a relatively high rate of transpiration ensues. 

 The soil is apt to be deficient in water at such times, or even if present, that 

 in the surface laj^ers may remain frozen during a period of warm air tempera- 

 tures. Even if neither of the foregoing conditions exists, the soil tempera- 

 tures will be low, and this greatly retards the rate of absorption of water. 

 The combined effect of a relatively high transpiration rate and relatively 

 low absorption rate results in a gradual desiccation of the leaves and branches 

 of the plant. This diminution in the water content of the aerial organs dur- 

 ing warm periods in winter is more severe in windy weather and is more likely 

 to occur if a sequence of mild days follows immediately after a cold spell. 

 If severe enough this desiccation will result in the death of some of the 

 branches, or in extreme cases of the entire tree or shrub. This is one cause of 

 the phenomenon known as winter- killing. Winter-killing due to desiccation 

 of the tissues should not be confused with cold injury due to low temperatures, 

 which is an entirely different phenomenon (Chap. XXXIII). 



