=143.= It has been shown elsewhere that the daily mean of temperature can be closely approximated from the maximum and minimum of both day and night. Maximum-minimum instruments for the other factors are lacking, however, and for light, humidity, and wind these values can only be obtained from the ecograph which makes it possible to get the exact mean from the sum of all the hour readings. When it comes to the seasonal mean, the ecograph is even more necessary, exception being made for water-content, in which case a number of readings on various days through the season will suffice. The value of factor means for diagnosis and for curves has already been sufficiently commented upon, and the feasibility of factor sums already indicated. CHAPTER III. THE PLANT STIMULUS AND RESPONSE GENERAL RELATIONS =144. The nature of stimuli.= Whatever produces a change in the functions of a plant is a stimulus. The latter may be a force or a material; it may be imponderable or ponderable; effect, not character, determines a stimulus. Consequently, reaction or response decides what constitutes a stimulus. The presence of the latter can be recognized only through an appreciable or visible response, since it is impossible to discriminate between an impact which produces no reaction and one which produces a merely latent one. From this it is evident that quantity is decisive in determining whether the impact becomes a stimulus. Plants grow constantly under the influence of many stimuli, all varying from time to time in amount. Small changes in these are so frequent that, in many cases at least, the plant no longer appreciably reacts to them. Such changes, though usually measurable, are not stimuli. Furthermore, it must be clearly recognized that plants which are in constant response to stimuli are stimulated anew by an efficient increase or decrease in the amount of any one of these. As is well known, however, such increase or decrease is a stimulus only within certain limits, and the degree of change necessary to produce a response depends upon the amount of the factor normally present. The entire absence of a force usually present, moreover, often constitutes a stimulus, as is evident in the case of light. The nature of the plant itself has a profound bearing upon the factors that act as stimuli. Many species are extremely labile, and react strongly to relatively slight stimuli; others are correspondingly stable, and respond only to stimuli of much greater force. Some light is thrown upon the nature of this difference by the behavior of ecads. A form which has grown under comparatively uniform conditions for a long time seems to respond less readily, and is therefore less labile than one which is subject to constant fluctuation. In many cases this is not true, however, and the degree of stability, i. e., of response, can only be connected in a general way with taxonomic position.