ALPINE AND TROPICAL PLANTS 159 



longer periods in the tropics, the total growth is much 

 greater, more rapid and luxuriant. 



Such results must be looked on as preliminary ; we are 

 in a position to appreciate how varied are the factors involved 

 in governing assimilation ; we scarcely yet know how 

 variation in leaf structure enters into the matter. Still less 

 do we understand the effect of possible changes in the minute 

 structure of the chloroplast. The investigation of the 

 relation of different plant types to efficiency in assimilation 

 is practically a virgin field. 



§ 18. Extent of Transpiration 



We come back now to some considerations regarding 

 transpiration, which also have an important bearing on 

 photosynthesis. We have already referred to the extent 

 to which transpiration takes place in ordinary plants, and 

 have seen that the amounts of water passing through the 

 plant are very large. Ganong has reckoned that the 

 transpiration of ordinary greenhouse plants ranges through 

 the day from 15 to 250 grm. per hour per square metre of 

 leaf surface, with an average of about 50 ; at night the 

 average falls to about 10. We may get a more vivid idea 

 of what this means by taking Hales' values — the first 

 obtained — for the transpiration of the sunflower. He found 

 that a large plant with a leaf area of 5616 sq. in. or 

 39 sq. ft., gave off on an average 20 ounces of water in 

 the 12 hours of a hot day, that is to say, about i pint ; 

 while a cabbage with one-half the leaf surface (2736 sq. 

 in.) gave off as much as i| pints. Von Hohnel reckoned 

 that a large birch, bearing about a quarter of a million leaves, 

 might give off over 400 kilos, of water on a sunny day, or 

 about 90 gallons. Such figures serve to emphasise the very 

 considerable water turnover of the leaf in a short time, 

 and the extent to which the root must draw on the soil. 

 Now it is not uncommon in hot sunny weather to see such 

 plants as the sunflower or the potato drooping in our gardens 

 and fields, or the red campion and foxglove in natural 



