168 BOARD OF AGRICULTURE. 



thousand on every square inch of the lower. These stomata 

 are furnished with openings so constructed as to close in very 

 dry air and open in that which is moist, but they always 

 remain shut except under the stimulus of light. As the chief 

 function of the rootlets is to absorb the liquid food of the 

 plant from the earth, so it is the special work of the stomata 

 to transpire the surplus water of the crude sap, which has 

 been employed as a carrier of food from one extremity of the 

 countless series of cells which build up the plant to the other, 

 in some cases a distance of five hundred feet through imper- 

 forate membranes and against the fotce of gravitation. 



In regard to the size of vegetable cells it is difficult to 

 obtain a correct conception, unless one is ftimiliar with the 

 use of the compound microscope. The spores or reproduc- 

 tive cells of some fungi, like the black smut on w^heat, are so 

 minute that eight millions of them placed side by side would 

 only cover one square inch of surface. In every cubic inch 

 of maple-wood there are probably not less than one hundred 

 million cells of the various tissues. The averao^e diameter of 

 ordinary plant-cells is less tliiln one four-hundredth of an 

 inch, and even the ducts or continuous tubes are not usually 

 much larger, and are often smaller than this. Professor 

 Gray informs us that sap must pass through two thousand 

 partitions in every inch of bass-wood through which it rises. 

 If the cells of the gigantic gum-tree of Australia are as small 

 as this, then a drop of sap must permeate about twelve 

 millions of membranous walls in passing from the rootlet by 

 which it is alisorbed to the topmost leaf by which it may be 

 exhaled. 



The vegetable kingdom has been created for the evident 

 purpose of establishing perpetual harmony between the min- 

 eral kingdom on the one hand and the animal kingdom on 

 the other. Plants produce all the food and all the vital air 

 which are indispensable to animals. Every growing plant 

 may be regarded as a machine for converting mineral matter 

 into cellulose, gum, starch, sugar and the various albuminoids 

 by the digestion and assimilation of which animals live and 

 grow. Every thriving plant is also exhaling continually, 

 under the stimulus of sunlight from its myriads of stomata, 

 pure vapor of water and oxygen gas, and we have often 



