No. 7. DEPARTMENT OF AGRICULTURE. 133 



of sap. If the soil is very damp and trauspiration from the plant 

 is slight, owing to a humid atmosphere, the root pressure is so great 

 that drops of water will be forced out at the points in the margin 

 of leaves. I have frequently observed this, particularly upon plants 

 in the greenhouse. 



The actual measurements of root pressure place more clearly be- 

 fore us the magnitude of the force. As early as 1720, the Reverend 

 Stephen Hale attached mercurial manometers to stumps of the 

 European grape and obtained a maximum pressure of 43 feet of 

 water. In 1874 W, S. Clark, of the Massachusetts Agricultural Col- 

 lege, made many tests, in which he obtained for the Black Birch a 

 pressure of 84.77 feet of water, and for a strong vine of our native 

 summer grape (Vitis aestivalis) a pressure sufficient to sustain a 

 column of water 88.74 feet in height. In a unique experiment per- 

 formed upon a mammoth squash by this same observer, the root 

 pressure of growth was measured in terms of pounds. The young 

 fruit was harnessed in two hemispheres of strap iron firmly riveted 

 together and a strong lever carrying weights w'as adjusted over the 

 cage to keep the hemispheres together as the squash was increasing 

 in size. As the tissue gained strength the weight was increased 

 until the total weight raised by the growing squash was 5,000 

 pounds or two and a half tons. Facts like these picture to us the 

 wonderful nature of the plants about us and turn us to thinking 

 how we may take advantage of this or that force, how we may aid 

 a plant to develop symmetrically and yield more abundantly the 

 crop we desire. 



Turning our attention for a moment to the third part of the plant 

 structure, the leaves, we learn by a microscopical study of their 

 structure that they are formed to do a vast and important work in 

 the economy of plant life. Transpiration is taking place from the 

 surface of the leaves; the w^ater, which was the carrier from the 

 roots to the leaves of the plant foods taken up in the soil, is being 

 evaporated from the combined area of the leaves. This area in the 

 average apple tree is an acre in extent and several tons of water are 

 evaporated by it in a single growing season. The leaves are termed 

 the "lungs of plants," performing that function for plants corre- 

 sponding to the breathing of animals, and to aid in this, the leaves 

 are possessed of thousands of breathing pores through which gases 

 pass into and out from the leaves. These breathing pores are found 

 upon all green parts of plants, and in some cases are so numerous 

 that 180,000 of them have been calculated to the square inch. The 

 unnatural loss of foliage is to a plant as hurtful as the loss of lungs 

 would be to the animal. Hence, we can understand why a peach 

 tree defoliated by the disease called Leaf-curl will drop all its 



