THE GENERAL CHEMISTRY AND PHYSICS OF PLANT- LIFE. 273 



ground from their low position, and may push their way out of 

 the leaf-sheaths. 



The following phenomena come under the same teleological 

 category. 1. Many plants show little or scarcely any growth in 

 the dark. 2. Boots, in general, grow more rapidly in the dark 

 than in the light. 3. Many fungi can grow only in the dark. 

 Of the latter we know that light is not a factor in their nutrition. 

 In the case of roots we know from observation that their normal 

 growth takes j)lace in the dark. In regard to 1, many cases 

 are yet not satisfactorily explained ; but it is very clear from 

 physiological evidence that many fern-spores require light for 

 their germination (Borodin and others) ; this is also true of the 

 spores of some mosses (Borodin, Leitgeb), In these cases the 

 products of germination can only continue their growth with the 

 aid of sunlight. 



Pliototonus is the term applied to the normal reaction of 

 plants to the rhythmic change of light and dark (day and night). 

 A longer or shorter exposure to dark will temporarily destroy 

 this phototonic condition, producing a transitory state of rigidity 

 (Dunkelstarre, Sachs). 



In general it may be stated that the phototonic plants, hence 

 normal plants, exposed to the daily change of light grow more rapidly 

 during the dark period (night), lohile uniform illumination {day) 

 retards thegroivth. Special cases with explanations have already 

 been cited. 



Intimately associated with pliototonus, that is, the condition 

 produced by alternating light stimuli, is the jDlienomenon of 

 " sleeping " and " waking " (opening and closing) of leaves ; in 

 other words, the nyctitropic movements. According to Charles 

 Darwin, the purpose of this movement is to reduce the radiation 

 of heat during cold nights. Among flowers it is usually a pro- 

 cess of closing, and also serves the function of protecting the 

 internal organs. These movements are not always curvatures, 

 but sometimes also torsions. We wall here consider only the 

 simple curvatures. 



The mechanics of this movement, of which the following are 

 the essential features, is only in part explained. Groivth processes 

 of the joints or motile organs, or of the nyctitropic organs with- 

 out motile organs, are not the only conditions producing this 

 phenomenon ; turgor-changes are the essential factors. In the 



