MECHANISM OF THE MOVEMENTS. 8oi 



fact that the petiole, which bends low down in the evening, should become erect in the 

 night, and why two elevations should also take place in the day. 



If we recall what has been said above as to the various combinations of the effects of 

 temperature and light indirectly on the entire plant as well as immediately on the con- 

 tractile organs, some such explanation as the following may be attempted of the diurnal 

 periodicity observed by Bert and Millardet. 



The strong depression of the petiole in the evening is caused by the effect of darkness 

 on the contractile organ ; it is doubtful whether the turgidity increases in it during the 

 depression ; but the amount of water in the plant increases during the night in conse- 

 quence of the diminished transpiration from the leaves. The contractile organs also 

 thus become more strongly charged with water, especially on their under side ; and 

 therefore gradually assume a more erect position. At sunrise, when the light should 

 properly cause the petiole to become still more elevated, transpiration increases and the 

 plant loses water, and the organs therefore also become more flaccid (again especially on 

 the under side), perhaps also in consequence of the action of the increase of temperature 

 directly on the contractile organ. But the roots, which at the end of the night had 

 become colder and less active, have in the meantime again become warm, and the more 

 powerful absorption fills the plant more completely with water, which causes the eleva- 

 tion of the petiole in the forenoon. But the gradual elevation of temperature causes a 

 fresh decrease in the quantity of water in the plant and in the contractile organs, and 

 perhaps directly causes the latter to assume the position of sleep. Hence a depression 

 takes place about noon ; but this is followed in the afternoon by a second elevation, 

 perhaps in consequence of the smaller amount of transpiration which results from the 

 fall in the temperature. Towards evening the light diminishes in intensity, inducing the 

 nocturnal position of the organs. Further researches may show how far this tentative 

 explanation, derived from a very defective knowledge of the causes of the movements, 

 may be adequate. 



3. With respect to the Mechanism of spontaneous periodic movements^ on which some- 

 thing has been said in Sect. 27, still less is at present known than with respect to those 

 connected with sleep. That we have here also to do with alternate elongation and con- 

 traction of the parenchyma of the upper and under sides of the organ is at once evident ; 

 and it is more than probable that this is also brought about essentially by increase and 

 diminution in the amount of water. But by what means the turgidity, first of one and 

 then of the other side of the organ, alternately increases and diminishes, while the 

 temperature, intensity of light, and amount of water remain constant, is as completely 

 unknown as why first one and then the other side of growing and revolving stems and 

 tendrils grows for the time most rapidly. 



CHAPTER VI. 

 THE PHENOMENA OF SEXUAL REPRODUCTION. 



Sect. 30. The essential element in the process of sexual reproduction 



is the formation, in the course of development of the plant, of cells of two different 

 kinds, which have no independent power of further development, but which, by 

 their coalescence, give rise to a product which possesses that power. 



It is only in a comparatively small number of cases, and in plants of very 

 simple structure, Hke the Desmidieae, Mesocarpeae, and Volvocinese, that the two 

 uniting cells are alike in their mode of production, size, form, and behaviour when 



3 F 



