He constantly notes small points of interest, e.g. (p. 82) that with cut branches the water absorbed diminishes each day, and that the former vigour of absorption may be partly renewed by cutting a fresh surface. [129a]
He also showed (p. 89) that the transpiration current can flow perfectly well from apex to base when the apical end is immersed in water.
These are familiar facts to us, but we should realise that it is to the industry and ingenuity of Hales that we owe them. In a repetition (p. 90) of the last experiment we have the first mention of a fact fundamentally important. He took two branches (which with a clerical touch he calls M and N), and having removed the bark from a part of the branch, dipped the ends in water, N with the great end downwards but M upside down. In this way he showed that the bark was not necessary for the absorption or transmission of water. [129b] I suspect
that one branch was inverted out of respect for the hypothesis of sap-circulation. He perhaps thought that water could travel apically by the wood, but only by the bark in the opposite direction.
Next in order (p. 95) comes his well-known experiment on the pressure exerted by peas increasing in size as they imbibe water. There are, however, pitfalls in this result of which Hales was unaware, and perhaps the chief interest to us now is that he considered the imbibition of the peas [130a] to be the same order of phenomenon as the absorption of water by a cut branch—notwithstanding the fact that he knew the absorption to depend largely on the leaves. [130b] It may be noticed that Sachs, in his imbibitional view of water-transport, may be counted a follower of Hales.
In order to ascertain “whether there was any lateral communication of the sap and sap vessels, as there is of blood in animals,” Hales (p. 121) made the experiment which has been repeated in modern laboratories, [130c] i.e. cutting a “gap to the pith,” and another opposite to it and a few inches above. This he did on an oak branch six feet long whose basal end was placed in water. The branch continued to “perspire” for two days, but gave off only about half the amount of water transpired by a normal branch. [130d] He does not trouble himself about
this difference, being satisfied of “great quantities of liquor having passed laterally by the gap.”
He is interested in the fact of lateral transmission in connexion with the experiment of the suspended tree (Fig. 24, p. 126), which is dependent on the neighbours to which it is grafted for its water supply. This seems to be one of the results that convinced him that there is a distribution of food material which cannot be described as circulation of sap in the sense that was then in vogue.
Hales (p. 143) was one of the first [131a] 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. [131b] 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.