GENERAL REQUIREMENTS. 9 



which eventually meet together and form a sharp, almost thorny 

 tip to the leaflet, thus making it also difficult for the wind to 

 start tearing the blade lengthwise (see illustration No. 12, 

 page 10&). As in the case of other trees there is a cuticle (skin) 

 on both the upper and lower surfaces of the leaflet, but inthe^case 

 of the date palm the cuticle is specially thick. The cuticle is 

 the outer layer of the outer wall of the external row of cells. It 

 is thickly impregnated with a substance known as " cutin " 

 which makes it tough and impervious to air and water. This 

 cuticle protects the more delicate cells lying under it to a greater 

 or less extent from abrasion from such bodies as wind-blown 

 sand grains, etc., and prevents these cells from being dried up by 

 dry air outside. 



In the cuticle, there are rows of very tiny openings or mouths 

 known as " stomata " through which the plant transpires its 

 excess of water in the form of vapour into the atmosphere and 

 also passes various gases necessary for its existence (see illustra- 

 tion No. 11, page 8b). These stomata are so tiny that a magni- 

 fication of nearly 300 diameters is required to examine their 

 structure a pin-prick is large in comparison. Like the stomata 

 in other plants, these open when their guard cells become turgid 

 with sap and close when they become flaccid. In this case, 

 however, they are sunken below the general level of the surface 

 of the leaf blade in small pits as is common only in xerophytic 

 (drought-resisting) plants. In plants the leaves of which lie 

 roughly horizontal, and therefore expose only their upper surfaces 

 to the sun's incident rays, the stomata are usually mostly confined 

 to the back (lower surface) of the leaf while the internal structure 

 shows the part near the face (upper surface) to be specially con- 

 structed to allow the chlorophyll granules (green colouring matter) 

 to collect the maximum energy from the sun's rays for the work 

 of food elaboration, tissue building, etc. 



In the date palm the stomata are about as numerous on the 

 face as on the back of the leaflet ; also there is no marked 

 difference in the internal structure of the leaflet towards one 



