FUNCTIONS 223 



been found to be able to endure a considerably higher 

 internal temperature than ordinary plants. Such plants 

 evidently cannot afford that loss of water which would be 

 involved in their cooling off by evaporation. Their ability 

 to endure high temperatures indicates that their proto- 

 plasm has power to withstand heat which is much greater 

 than that usually possessed by protoplasm. 



In this connection we see that transpiration may be a 

 disadvantage on one day and an advantage on the next. 

 Thus on a hot dry day after a long drought the leaves of 

 corn may be in danger of drying up. They are losing far 

 more water by evaporation than is needed to keep the in- 

 side of the leaf from overheating or for any other purpose. 

 Under such conditions excessive transpiration certainly 

 seems a disadvantage. Yet the very next day may be hot 

 and moist and the leaves will be needing all the transpira- 

 tion that is possible in order to keep them from overheat- 

 ing. Evidently we cannot judge fairly of the advantages 

 or disadvantages of any feature of plant life without tak- 

 ing into consideration the changes in external conditions 

 which are constantly occurring. 



Another and a very decided " advantage " of transpira- 

 tion is that it is a necessary part of the process whereby 

 the ascent of water and solutes is accomplished. You know 

 that these solutes are necessary for the work of the leaf. 

 You have already learned that their entrance into the root 

 from the soil depends upon the presence of water, but not 

 upon its movement (see page 109) ; you have noted that into 

 living cells and through them these solutes move in accord- 

 ance with the laws of osmosis and are not carried along in a 

 mass movement of water. But by far the greatest part of 

 the journey from root to leaf is not through living cells. It 



