Page:Makers of British botany.djvu/104

 74 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 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.

An experiment that I do not remember to have seen quoted elsewhere is worth describing. It is one of the many experiments that show the generous scale on which his work was planned. An apple bough five feet long was fixed to a vertical glass tube nine feet long. The tube being above and the branch hanging below the pressure of the column of water would act in concert with the suck of the transpiring leaves instead of in opposition to this force. He then cut the bare stem of his branch in two, placing the apical half of the specimen (bearing side branches and leaves) with its cut end in a glass vessel of water, the basal and leafless half of the branch remained attached to the vertical tube of water. In the next 30 hours only 6 ounces dripped through the leafless branch, whereas the leafy branch absorbed 18 ounces. This, as he says, shows the great power of perspiration. And though he does not pursue the experiment, it is worthy of note as an attempt like those of Janse and others to correlate the flow of water under pressure with the flow due to transpiration.

It is interesting to find that Hales used the three methods of