240 TEXTBOOK OF BOTANY 



trees in the gorges are living in a humid "microchmate" where drought 

 rarely becomes sufficiently excessive to inhibit their development. The 

 undergrowth in such situations is just as different from that on the 

 south slope and the upland as are the dominant tree species. 



Transpiration in relation to available soil water will be discussed fur- 

 ther in Chapter XXX. 



Major vegetation types and transpiration. Transpiration when studied 

 in the laborator\' and greenhouse is often compared with evaporation 

 from a wet surface. Under the experimental conditions the curves for 

 evaporation and transpiration are frequently very similar. One may 

 therefore be led to the conclusion that where the rate of evaporation 

 of water is high in nature, the transpiration will also be high. This 

 reasoning overlooks the matter of water supply upon which all transpira- 

 tion is dependent. Forests, prairies, steppes, semi-deserts, and deserts 

 occur in regions of successively increasing drought and increasing 

 evaporation from water surfaces. The annual evaporation may be 50 

 times as great from an open pan of water in the desert as from a similar 

 pan in some wet forest region. The annual transpiration from desert 

 plants, however, is very slight when compared with that of plants in a 

 wet forested region. 



Transpiration in nature decreases as one goes from forest to prairie, 

 to steppe, to semi-desert, and to desert for the obvious reason that the 

 plants in these formations have successively decreasing amounts of water 

 available. Their periods of greatest loss of water are more and more 

 limited to moist periods instead of throughout the year. The date palm 

 cited earlier in this chapter illustrates how great transpiration may 

 become in some plants when there is an unlimited water supply. The 

 cactus exemplifies how low it may be when the water supply is reduced 

 to a minimum. An acre of forest may lose 10 to 25 acre-inches of water 

 in moist temperate regions, while an acre of prairie grass in Nebraska 

 loses about 2 acre-inches. If transpiration of trees were as effectively 

 checked as in certain cacti without interfering with photosynthesis and 

 respiration, the whole landscape east of the Rockies would be domi- 

 nated by forests. 



Is transpiration an essential process? Transpiration is an inescapable 

 physical process in most species of plants. Is it merely neutral, or harm- 

 ful, or is it an essential process comparable to photosynthesis and respira- 

 tion? This question has often been discussed pro and con in botanical 

 literature, and it is frequently answered without a clear analysis of the 



