3^4 



evaporation due to direct insolation, tend to increase the amount of 

 water available for the plant. 



Other conditions being equal, it may be regarded as a fact that 

 water plays a more important part in determining the yield of a 

 crop, than manure or any other essential of plant life ; and one has 

 only to compare the growth and returns from coffee and other pro- 

 ducts grown on hill and alluvial soil respectively, or of padi grown 

 on dry and on irrigated land, to see how true this is of this part of 

 the tropics. 



The whole of the plant food obtained from the soil must enter 

 the plant in a liquid state, as it is only in this condition that it can 

 pass through the cell walls ; and as the watery fluid taken up by the 

 roots contains but a very minute quantity of plant food, the plant is 

 compelled to take up more water than is necessary for its imme- 

 diate requirements, in order to secure a sufficient quantity of the 

 various salts and oxides which are held in solution. The surplus 

 water thus absorbed, simply as a medium for the transportation of 

 the plant food, is transpired through the leaves, and, though to a 

 less extent, through the stem and other parts of the plant : thus a 

 current of water known as the transpiration current is maintained, 

 fresh supplies passing into the plant through the roots, and passing 

 up through the wood cavities, to make good the loss due to trans- 

 piration. If for any reason the supply of water is insufficient to 

 compensate for the loss due to transpiration, the plant wilts ; and it 

 is due to this fact that a cut branch withers and dies. 



Transpiration is promoted by numerous small openings in the 

 epidermis of the leaves, known as stomata. These stomata are 

 most generally found on the under surface of the leaf, though in 

 some plants where the leaves are placed more or less vertically, as 

 in the Yucca, they occur in about equal numbers on both the under 

 and the upper surface. These pores though very minute, are 

 usually present in enormous numbers; it being estimated that 

 160,000 occur within the space of a square inch of the under sur- 

 face of the leaf of the Lilac ; while an ordinary Sunflower leaf is 

 provided with no less than 13,000,000. 



It will be seen therefore, that though the stomata are so minute 

 that liquid water cannot pass through them, evaporation in the 

 form of a watery vapour is greatly facilitated, owing to the enor- 

 mous numbers in which they occur ; a large number of small open- 

 ings being much more effective than a small number of large ones. 



The amount of transpiration however, can be regulated, and 

 where the external conditions are such that active transpiration 

 would be injurious to the plant, the stomata can be closed and 

 transpiration thereby checked ; each stoma being provided by two 

 guard cells which, when acted upon by various stimuli, have the 

 power of closing the pore. 

 (To be continued.) 



STANLEY ARDEN, 

 Superintendent, Experimental Plantations, 

 S elan go r . 



