Hales (p. 143) was one of the first[44] to make the well-known experiment—the removal of a ring of bark, with the result that the edge of bark nearest the base of the branch swells and thickens in a characteristic manner. He points out that if a number of rings are made one above the other, the swelling is seen at the lower edge of each isolated piece of bark, and therefore (p. 143) the swelling must be attributed "to some other cause than the stoppage of the sap in its return downwards," because the first gap in the bark should be sufficient to check the whole of the flowing sap[45]. He must in fact have seen that there is a redistribution of plastic material in each section of bark.
We now for the moment leave the subject of transpiration and pass on to that of root-pressure on which Hales is equally illuminating.
Figure from Vegetable Staticks showing a vine with mercury gauges in place to demonstrate root-pressure.
His first experiment, Vegetable Staticks, p. 100, was with a vine to which he attached a vertical pipe made of three lengths of glass-tubing jointed together. His method is worth notice. He attached the stump to the manometer with a "stiff cement made of melted Beeswax and Turpentine, and bound it over with several folds of wet bladder and pack-thread." We cannot wonder that the making of water-tight connexions was a great difficulty, and we can sympathise with his belief that he could have got a column more than 21 feet high but for the leaking of the joints on several occasions. He notes the familiar fact that the vine-stump absorbed water before it began to extrude it.
He afterwards (pp. 106-7) used a mercury gauge and registered a root-pressure of 32½ inches or 36 feet 5⅓ inches of water which he proceeds to compare with his own determination of the blood-pressure of the horse (8 feet) and of other animals. Perhaps the most interesting of his root-pressure experiments was that (p. 110) in which several manometers were attached to the branches of a bleeding vine and showed a result which convinced him that "the force is not from the root only, but must proceed from some power in the stem and branches," a conclusion which some modern workers have also arrived at. The figure on [page 77] is a simplified reproduction of the plate (Fig. 19) in Vegetable Staticks.
Assimilation.
Hales' belief that plants draw part of their food from the air, and again that air is the breath of life, of vegetables as well as of animals (p. 148), are based upon a series of chemical experiments performed by himself. Not being satisfied with what he knew of the relation between "air" (by which he meant gas) and the solid bodies in which he supposed gases to be fixed, he delayed the publication of Vegetable Staticks for some two years, and carried out the series of observations which are mentioned in his title-page as "An attempt to analyse the air, by a great variety of chymio-statical experiments" occupying 162 pages of his book[46].