156 TRANSPIRATION. 



prairies, which has its leaves placed vertically, 

 with the edges pointing north and south, so that 

 the plant is said to guide travellers on their jour- 

 neys across the prairie. 1 At morning and evening 

 the leaves are warmed directly by the sun's rays, 

 but at midday, when the sun is hot overhead, the 

 beams fall on the edges of the leaves, and the 

 plant is protected from too great transpiration. 

 Young leaves, by taking a vertical position, are 

 protected from exposure to the direct rays of the 

 sun. 2 



The leaves of plants which float on the surface 

 of the water, as the Water Lily, have their stomata 

 all on the upper side. Submerged water-plants 

 have no stomata at all, nor are the outer cells of 

 their epidermis waterproof. They transpire from 

 their whole surface, and this is the reason why they 

 collapse so immediately when they are taken from 

 the water. Their food is all about them, and they 

 have no need for such a complicated arrangement 



1 Longfellow mentions this in " Evangeline " : 



" Look at this delicate plant that lifts its head from the meadow, 

 See how its leaves all point to the north, as true as the magnet; 

 It is the compass flower, that the finger of God has suspended 

 Here on its fragile stalk to direct the traveller's journey 

 Over the sea-like, pathless, limitless waste of the desert." 



He is in error, however, in calling the plant delicate. 



2 See VIII. " Young and Old Leaves," p. 84. 



