8 



THE COCO-NUT 



CHAP. 



evaporating from a tea-kettle leaves behind it the salts 

 it held in solution, forming a shell on the inside of the 

 kettle ; and as the water flowing in through the rivers 

 has evaporated and left the ocean salt; so the water 

 which evaporates from the coco-nut leaves behind it 

 what it held in solution, and this residue makes up the 

 whole of the mineral and nitrogenous food of the tree. 

 The more water is boiled in the kettle, the more residue is - 

 left ; and the more water evaporates from the leaves, the 

 more mineral and nitrogenous food remains for the plants. 

 Men cultivate the soil and thereby directly and indirectly 

 increase the amount of food the plant can get from it. 

 This costs money. They buy fertilizers and apply 

 them to the soil. This also costs money. Exactly the 

 same end can often be secured with little or no expense 

 by increasing the evaporation from the leaves ; which 

 gives the subject a very evident practical importance. 



The evaporation of water from plants is called trans- 

 piration. It takes place in part directly through the 

 outer walls of the epidermis, but chiefly through small 

 openings in the epidermis called stomata. The stomata 

 of the coco-nut are entirely confined to the nether sur- 

 faces of the leaves. At least 98 per cent of the water 

 transpired is given off from this surface. The stomata 

 open to their full width in the day-time, in full sunshine, 

 provided that the tree is well supplied with water. If 

 the tree is not thus well supplied, the openings narrow 

 as the water in the leaves decreases, and so check the 

 transpiration. They are closed at night. The trans- 

 piration will be most active if the roots can take up 

 water and supply it to the leaves as fast as it is 

 evaporated, and the stomata thus be kept wide open 

 throughout the day. 



Besides the stomata, the coco -nut has another 

 structure which regulates the transpiration, but less 

 perfectly. This is a strand of tissues running along 

 each side of the midrib, on the under side of the pinna, 

 and acting as a hinge. The pinna is thinnest along 

 these lines. The cells of the hinge are colourless and 



