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 96 Hill had considerable technical ability and, I think, was capable of greatly advancing anatomical botany; unfortunately, however, he gave too little time and thought to his investigations.

Physiology.

The eighteenth century saw the birth of vegetable physiology, Hales and Knight being the two great pioneers in this country. The former flourished in the early part of the century, whilst Knight, although born in 1758, published his great work in 1806.

The chief physiological work of Hill is embodied in a pamphlet of 59 pages, entitled The Sleep of Plants and Causes of Motion in the Sensitive Plant explain'd, published in London in 1757, a year previous to the appearance of Du Hamel's Physique des Arbres. The paper is in the form of a letter to Linnaeus, and in it the author explains his position with regard to his earlier criticisms of the Linnaean system of classification.

The work is divided into sections, the first of which consists of a brief historical resumé, the opinions of Acosta, Alpinus, Ray and Linnaeus on this subject being alluded to. No mention, however, is made of the observations of Bonnet and of Mairan to the effect that the periodic movements of Mimosa pudica continued when the plant was kept in prolonged darkness.

In Section 2, after describing the structure of a leaf, Hill remarks that "Leaves are always surrounded by the air; and they are occasionally and variously influenced by heat, light, and moisture. They are naturally complicated, and they act on most occasions together. We are therefore to observe, first, what effects result from their mutual combinations in a state of nature: and having assigned in these cases the effect to the proper and particular cause, from this power of that agent, whichsoever it is, that acts thus in concert with the rest, we may deduce its operations singly."

This passage, although not particularly clear, indicates that Hill fully appreciated the fact that the reaction exhibited by a plant organ is a response to the resultant of a number of forces, and that each factor must be examined separately.