On a Method of Studying Transpiration
F. Darwin
1914
Proceedings of the Royal Society of London. Biological Sciences
Transpiration is, perhaps, more directly under the rule of external physical conditions than any other physiological function. Yet proofs of this conclusion are wanting, at any rate in regard to the transpiration of leaves. Thus, as far as I know, we have no complete experimental determination of the relation between the loss of water-vapour from leaves and the relative humidity of the air. Nor again have we any complete evidence as to the effect on transpiration of variation in the
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... to which the leaf is subjected. These lacunae in our knowledge depend on the fact that in leaves, tran spiration is largely dependent on the behaviour of the stomata, being relatively large when they are wide open, and diminishing as they close. And since the aperture of the stomata depends on external condition, it is clear that no distinction can be made between the diminution in evaporation resulting from increased relative humidity of the air, and the diminution in the transpiration-rate due to stomatal closure. In fact it is impossible to learn anything accurately concerning transpiration until the varying aperture of the stoma is excluded from the problem. This might possibly be done by estimating the transpiration of leaves of aquatic plants in which the stomata vary but slightly in aperture; but the experiment would not be easily made in a trustworthy form. The method I have actually employed is to block the stomata with a fatty substance,* and then to place the intercellular spaces of the leaves in com munication with the external air by means of incisions. Most of the experiments were made on laurel (P. laurocerasus). The lower surface of the leaf was smeared with melted cocoa-butter or with vaseline rubbed in with the finger, and four to six cuts were made with scissors or a razor, reaching from the periphery to the midrib between the large veins. Other plans were also tried, e.g. pricking the leaf with a needle or making numerous small incisions by stabbing with a scalpel.f The method is similar to that of Stahl,J who showed that greased leaves pierced with holes assimilate and form starch in the tissues surrounding * Cocoa butter in the earlier experiments, vaseline in all the later ones. + The method was described in a paper read at Section K of the Sheffield meeting of the British Association, 1910 (title alone published).
doi:10.1098/rspb.1914.0011
fatcat:d7kpfrm435bevi3xdjmefyg6mq