272 TRANSFER OF WATER THROUGH THE PLANT. 



transpired from a vigorous sunflower, tliree feet and a lialf iiigh, 

 during twelve hours of a very warm da^-, was one pound four- 

 teen ounces, and, on an average, one pound four ounces was 

 transpired every twelve hours. Anj' evaporation from the sur- 

 face of the soil in the flower-pot in which the plant was growing 

 was prevented by a lead cover. 



A still simpler method of preventing evaporation is to en- 

 velop the flower-pot with a thin rubber meml»rane, and tie this 

 tightly around the stem of the plant. A fresh supply of water 

 can be given to the plant at any time by means of a tube close 

 to the stem. In expeiiments upon transpiration the plant should 

 be weighed frequentlj-, care being taken to note all the external 

 conditions, such as light, moisture of the atmosphere, etc. For 

 weighing, an open balance with large pans should be used. The 

 form known as the box scale will answer all ordinary purposes ; 

 but for delicate weighings one of special construction, having a 

 long beam, is preferable. 



hours day, which heing allowed in the daily weighing of the plant and pot, I 

 found the greatest perspiration of twelve hours in a very warm dry day, to he 

 one pound fourteen ounces ; the middle i-ate of perspiration one pound four 

 ounces. The perspiration of a dry warm night, without any sensihle dew, was 

 about three ounces ; but when any sensible, tho' small dew, then the per- 

 spiration was nothing ; and when a large dew, or some little rain in the night, 

 the plant and pot was increased in weight two or three ounces. N. B. T?ie 

 weights I made use of were Avoirdupoise weiyhts. 



" I cut off all the leaves of this plant, and laid them in five several parcels, 

 according to their several sizes, and then measured the surface of a leaf of each- 

 parcel, by laying over it a large lattice made with threads, in which the little 

 sq^uares were J of an inch each ; by numbering of which 1 had the surface of 

 the leaves in square inches, which multiplied by the number of the leaves in 

 the corresponding parcels, gave me the area of all the leaves ; by which means 

 I found the surface of the whole plant, above ground, to be equal to 5616 

 square inches, or 39 square feet. 



" I dug up another Sun-Flower, nearly of the same size, which had eight 

 main roots, reaching fifteen inches deep and sideways from the stem : It had 

 besides a very thick bush of lateral roots, from the eight main roots, which ex- 

 tended every way in a Hemisphere, about nine inches from the stem and main 

 roots. 



" In order to get an estimate of the length of all the roots, I took one of the 

 main roots, with its laterals, and measured nnd wei",'bed them, and then 

 weighed the other seven roots, with their laterals, by which means I found the 

 sum of the length of all the roots to be no less than 1448 feet. 



" And supposing the periphery of these roots at a medium, to be ?8 "f "" 

 inch, then their surface will be 2286 S(juare inches, or 15.8 square feet ; that 

 is, equal to § of the surface of the plant above ground " (Vegetable Staticks, 

 2d 9d., 1731, vol. i. p. 4). 



