General Physiology of the Plant. 209 



number of others secreted, or excreted by the plant, into 

 the discussion of which, however, we cannot enter here. 

 Many of these bodies, such as the aromatic substances, the 

 colouring matters of flowers, the vegetal alkaloids, perform 

 very important services in metabolism. Nectar (p. 169) 

 and the odorous substances employed in cross-fertilisation 

 in the manner already explained, are instances in point. 

 For a complete discussion of the subject of waste pro- 

 ducts, reference must be made to Vines' Lectures on Plant 

 Physiology^ lect. xii. 



Results of metabolism. We have lastly to glance at 

 the results of metabolism, which are chiefly expressible in 

 terms of the law of the conservation of energy. We may 

 consider these results under two headings : first, those 

 connected with the evolution of heat, light, and electricity ; 

 and secondly, those which are related to growth and move- 

 ment. Connected with the phenomena of movement may 

 be considered those of sensitivity, or irritability. 



Heat. Heat, we have already seen, is the ultimate form 

 into which all kinetic energy becomes transformed (p. 14). 

 The energy evolved by the plant is very considerable, and all 

 forms of it very rapidly undergo degradation into the final 

 form of energy. The surface of a plant, however, exposed 

 to the air is very great, so that the heat evolved is very 

 speedily dissipated. Of couise, where the greatest metabo- 

 lism is going on, as, for instance, in the reproductive organs, 

 in~ the growing shoots, and in germinating seeds, there the 

 greatest amount of heat is evolved. Unless specially pro- 

 tected, however, the temperature of a plant seldom rises 

 above that of the surrounding air, for the reason above 

 stated. It is more frequently beneath the temperature of 

 the air, owing to the cooling effect of evaporation from the 

 exposed parts. 



Light. The manifestation of light by plants, often 

 known as phosphorescence, is a phenomenon exhibited only 

 by the lower plants, such as some of the Fungi and Algae. 



p 



