Page:Makers of British botany.djvu/103

 Rh He goes on to measure the surface of the roots and to estimate the rate of absorption per area. The calculation is of no value, since he did not know how small a part of the roots is absorbent, nor how enormously the surface of that part is increased by the presence of root-hairs. He goes on to estimate the rate of the flow of water up the stem; this would be 34 cubic inches in 12 hours if the stem (which was one square inch in section) were a hollow tube. He then allowed a sunflower stem to wither and to become completely dry, and found that it had lost $3/4$ of its weight, and assuming that the $1/4$ of the "solid parts" left was useless for the transmission of water he increases his 34 by $1/3$ and gives $45 1/3$ cubic inches in 12 hours as the rate. But the solid matter which he neglected contained the vessels and he would have been nearer to the truth had he corrected his figures on this basis. The simplest plan is to compare his results with those obtained by Sachs in allowing plants to absorb solutions of lithium-salts. If the flow takes place through conduits equivalent to a quarter of a square inch in area, the fluid will rise in 12 hours to a height of 4 x 34 or 136 inches or in one hour to 28.3 cm. This is a result comparable to, though very much smaller than, Sachs' result with the sunflower, viz. 63 cm. per hour.

The data are however hardly worth treating in this manner. But it is of historic interest to note that when Sachs was at work on his Pflanzenphysiologie, published in 1865, he was compelled to go back nearly 140 years to find any results with which he could compare his own.

We need not follow Hales into his comparison between the "perspiration" of the sunflower and that of a man, nor into his other transpiration experiments on the cabbage, vine, apple, etc. But one or two points must be noted. He found the "middle rate of perspiration" of a sunflower in 12 hours of daylight to be 20 ounces, and that of a "dry warm night" about 3 ounces;