Page:Collected Physical Papers.djvu/346

326 by turning the rectangular plant chamber round a revolving base.

The photo-voltaic cell thus constructed is stimulated by light from an arc lamp which passes through a trough of alum solution for absorption of the heat rays. Successive exposures are made for 10 seconds and records obtained on a moving photographic plate. The normal responses are uniform, exhibiting induced galvanometric negativity as seen in the up-curves. On the cessation of light there is a complete recovery; the recovery shows in fact, an overshooting towards galvanometric positivity from which it returns almost to the original zero position before stimulation (fig. 97). The records indicate the existence of dual reactions, a negative variation or D-effect followed by a positive variation or the A-effect.

Some observers have obtained with green leaves a response of galvanometric positivity. The results of following investigation offer an explanation of the apparent anomaly. I have shown elsewhere that a positive response occurs under a stimulus below the critical intensity, and that this critical point is low in highly excitable tissues, whereas it is relatively high in others which are less excitable. The excitability, I find, is modified by the age and vigour of the specimen. It is very considerable at moderately young age, being feeble when the tissue is either very young or very old. The same stimulus which evokes a negative response in a vigorous middle-aged leaf may, therefore,