14 COCONUT PLANTER'S MANUAL. 



THE LEAF. 



The water taken up by the cells of the absorbent region of the 

 roots travels from cell to cell into the vessels that are found in the 

 central vascular bundles along which it ascends till it reaches the 

 stem and finally the leaves. There, travelling along the midrib and 

 veins and through the leaf tissue, it finds its way out by the stomata. 

 As a result of this evaporation (transpiration) there is left in the 

 leaves a residue consisting of the mineral (including nitrogenous) food of 

 the tree derived from the soil. The stomata are small openings in the 

 epidermis of the leaf and, in the coconut, occur only on the under sur- 

 face. Provided the tree is well supplied with moisture the stomata are 

 full open in the daytime when the sun shines. If not well supplied, 

 they narrow down as the water in the leaves decrease, so as to reduce 

 transpiration. At night they are closed. Transpiration will 

 be most active if the roots supply water as fast as it is evaporated ; and, 

 as a result, the tree will get the maximum of plant food from the soil. 



About 98 per cent of the water evaporated passes out through 

 the stomata of the leaves ; but transpiration is also regulated by a 

 strand of tissue which runs along the sides of the midrib on each 

 half of the leaf blade, and acts like a hinge. When the leaflets are well 

 stored with water, the cells of the hinge, which are colourless and 

 delicate, become turgid or swollen and cause each side of the leaflet 

 to expand. With a poor supply of water the cells of the hinge 

 become flaccid or slack, the two parts of the leaf blade collapse and 

 present a smaller surface to the sun, so that there is less evaporation. 

 Transpiration is increased by light, warmth, a dry atmosphere as well 

 as by wind. It is hindered by darkness, cold, a moist condition and 

 a still atmosphere. The most important factor influencing tanspira- 

 tion is light, and for this reason the tree must be fully exposed to 

 it. Encroaching shade from other trees, too closely planted palms, 

 high weeds, etc., is objectionable. In the hottest part of the day, 

 or during drought, the closing down of the two sides of the leaflets is 

 an indication that the quantity of water taken up by the roots is less 

 than what is needed for transpiration, which is, therefore, reduced by 

 the automatic action of the hinge lessening the surface of the 

 leaflets expose 1 to the sun. What ultimately regulates the amount of 

 water transpired is the amount that can reach the leaves. (The leaf 

 can also, when necessary, draw upon the moisture within the plant, 



