IX. On the relation between transpiration and stomatal aperture

Abstract

The subject of the present paper was briefly discussed in a former communication. The porometer there described was used to estimate changes in stomatal aperture by observing the variations in the rate of flow of an air current drawn through the stomata of an uninjured leaf under a given pressure. In 3, p. 139, the porometer was compared with Stahl’s cobalt method and my own horn hygroscope. It is superior to both these, which only give indirectly any information about the stomatal aperture. It is true that if the stomata close, e. g. in darkness, the readings of the horn hygroscope diminish, but, since transpiration is diminished by darkness quite apart from stomatal closure, the lower reading of the hygroscope may, indeed must, be partly independent of the closure of the pores. The same objection applies to the cobalt method, valuable though it is. The use of the porometer is in one way superior to Lloyd’s microscopic method, which consists in measuring the stomata in strips of cuticle forcibly removed from the leaf and fixed in absolute alcobol, since the porometer gives information about the living uninjured stomata. On the other hand, it does not tell us the dimensions of the stomata: it only gives variations in the rate of flow of air through these openings, which changes, however, must depend in some way on changes in the stomatal apertures. In our original paper the use of the porometer was illustrated in various ways: the closure of the stomata in darkness and their re-opening in light; the effect of withering, i. e. the preliminary opening of the stomata followed by closure, etc. The problem which forms the subject of the present paper, i. e. the relation between stomata and transpiration, is but briefly illustrated in the paper. Only a single experiment was given to show the parallelism which occurs between the variations in aperture of the stomata and in the rate of transpiration. In the present essay I hope to establish the fact that this parallelism holds good within certain limits. There are, indeed, frequent irregularities in the experiments, as disturbing causes cannot be avoided; but when the general correspondence between the results of a large number of experiments is considered, it seems to me impossible to accept Lloyd’s dictum with regard to stomata—that “their regulatory function is almost nil ." It must in justice be said that he elsewhere modifies this sweeping assertion. He allows (p. 35) that “complete closure of the stomata, if this occurs, reduces transpiration to, or nearly to, cuticular rate.” He also grants that the stomata give the upper limit of transpiration; that is to say, when the stomata are open to their utmost limit the highest rate of transpiration under these conditions is the maximum of which the leaf is capable. Anyone who has been familiar, as I have, with the general parallelism in the curves of transpiration and stomatal aperture in obedience to changing conditions cannot possibly give in to what must be called the illogical position just referred to, that in the intermediate conditions, when the stomata are neither shut nor widely open, the regulation is not chiefly stomatal but depends rather on other factors. I can only suppose that there is some flaw in Mr. Lloyd’s method which has escaped even his careful and conscientious manner of attacking the problem. He gives his readers every chance of estimating the accuracy of his results. The present paper shows the unavoidable uncertainties which attend the subject, and no one can read Mr. Lloyd’s account of the matter without realising the pains he took to get a trustworthy result.