The dedication is to George Prince of Wales, afterwards George III. The author cannot quite avoid the style of his day, for instance: "And as Solomon the greatest and wisest of men, deigned[28] to inquire into the nature of Plants, from the Cedar of Lebanon, to the Hyssop that springeth out of the wall. So it will not, I presume, be an unacceptable entertainment to your Royal Highness," etc.
But the real interest of the dedication is its clear statement of his views on the nutrition of plants. He asserts that plants obtain nourishment, not only from the earth, "but also more sublimed and exalted food from the air, that wonderful fluid, which is of such importance to the life of Vegetables and Animals," etc. We shall see that his later statement is not so definite, and it is well to rescue this downright assertion from oblivion.
His book begins with the research for which he is best known, namely that on transpiration. He took a sunflower growing in a flower-pot, covering the surface of the earth with a plate of thin milled lead, and cemented it so that no vapour could pass, leaving a corked hole to allow of the plant being watered. He did not take steps to prevent loss through the pot, but at the end of the experiment cut off the plant, cemented the stump and found that the "unglazed porous pot" perspired 2 ozs. in 12 hours, and for this he made due allowance.
The plant so prepared he proceeded to weigh at stated intervals. He obtained the area of the leaves by dividing them into parcels according to their several sizes and measuring one leaf[29] of each parcel. The loss of water in 12 hours converted to the metric system is 1·3 c.c. per 100 sq. cm. of leaf-surface; and this is of the same order of magnitude as Sachs' result[30], namely 2·2 c.c. per 100 sq. cm.
He goes on to measure the surface of the roots [31] 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 ¾ of its weight, and assuming that the ¼ of the "solid parts" left was useless for the transmission of water he increases his 34 by ⅓ and gives 45⅓ 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[32] 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 × 34 or 136 inches or in one hour to 28·3 cm.[33] 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[34] 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; thus the day transpiration was roughly seven times the nocturnal rate. This difference may be accounted for by the closure of the stomata at night.
Hales of course knew nothing of stomata, but it is surprising to find Sachs in 1865 discussing the problem of transpiration with hardly a reference to the effect of stomatal closure.
Hales[35] notes another point which a knowledge of stomatal behaviour might have explained, viz. that with "scanty watering the perspiration much abated," he does not attempt an explanation but merely refers to it as a "healthy latitude of perspiration in this Sunflower."
In the course of his work on sunflowers he notices that the flower follows the sun, he says however that it is "not by turning round with the sun," i.e. that it is not a twisting of the stalk, and goes on to call it nutation which must be the locus classicus for the term used in this sense.