Ch. Ill, 6] TRANSPIRATION FROM PLANTS 



51 



Therefore the capacity of the small stomatal openings for 

 gas passage is far in excess of that implied by their areas. 

 The matter becomes clearer from another point of view when 

 we note that an ordinary stoma when open presents to a 

 molecule of carbon dioxide or water an entrance or exit as 

 great as a passage seven miles wide appears to a man. 



While transpiration is thus primarily an incidental accom- 

 paniment of photosynthesis, rather than a physiological pro- 

 cess in itself, it does have 

 functional value in one 

 respect. Plants need in 

 their leaves, and else- 

 where, certain mineral 

 matters which are ab- 

 sorbed from the soil; 

 and these are lifted with 

 the water, and left in the 

 tissues by its evapora- 

 tion. Indeed, the view 

 has been held in the past 

 that this is the primary 

 functional meaning of 

 transpiration, its copi- 

 ousness being considered 

 necessary because of the 

 great dilution of the minerals in the soil water. Later evi- 

 dence, however, shows that little relation exists between the 

 amount of transpiration and the quantity of mineral matters 

 found in the plant. Furthermore, an important role has 

 been assigned to transpiration in the dissipation of the exces- 

 sive energy poured into leaves at times by the strongest 

 summer sun, — an amount sufficiently great to work damage 

 in the leaf were it not for the cooling effect of evaporation ; 

 :ind this advantage must be real, even though incidental 

 rather than adaptive. Thus it seems clear that transpira- 

 tion is primarily an unavoidable though partially controlled 



Fig. 23. — Diagram to show the num- 

 ber, and extreme area of opening, of 

 stomata, according to the conventional 

 constant; drawn to scale, 100 times the 

 true length and breadth. 



