INVERTEBEATA, CRTPTOGAMIA, MICROSCOPY, ETC. 751 



4. The lamina of leaves does not absorb water wbeu (1) they have 

 retained their tiirgidity ; at least it does not take place in the vicinity 

 of organs which are actively transj^iring ; (2) when they have within 

 their reach tissues containing abundance of water ; when slightly 

 withered they appear to prefer imbibing water from this source. 



Movements of Growing Leaves and Petals.* — Dr. Carl Eraus 

 has an elaborate and important paper on this subject in ' Flora.' The 

 author mentions in the first place that these periodic movements are 

 of two kinds : — those caused by unequally accelerated or retarded 

 growth of different masses of cells ; and those which are not dependent 

 on growth, but on a transitory alternate elongation and shortening of 

 certain masses of cells. The first kind are movements of nutation ; 

 to the second kind, which Pfefier has made specially his study, he has 

 applied the term movements of variation. His investigations have 

 shown that every sensitive movement caused by decrease of light is 

 followed by changes in position of the part affected, the extent of the 

 oscillations depending on the degree of change in the amount of 

 light. Alterations in the amount of light are, therefore, the chief 

 cause of those movements. A secondary cause is a change in the 

 amount of moisture in the air. On these two causes depend mainly 

 the opening and closing of flowers. It is obvious that the " bifacial " 

 structure of the flat, horizontal leaves of most dicotyledonous plants, 

 combined with the fact that, as the leaf unfolds, the under surface is 

 more completely exposed to the light than the upper surface, must 

 bring about different conditions in these respects between the two 

 surfaces of the leaf. The following are some of the special 

 phenomena thus exhibited : — 



1. Changes of position from decrease of turgidity. This is the 

 ordinary familiar phenomenon of the withering of leaves or petals 

 when the supply of moisture is cut off. In many plants, such as 

 Chenopodium album, Stellaria media, Nicotiana latissima, &c., the loss 

 of water is sjn'ead so rapidly from cell to cell that no perceptible 

 difference is manifested between the upper and lower surfaces, and 

 the leaves simply hang down ; and the same is the case with the 

 petals of Solanum tuberosum and Convolvulus arvensis ; while those of 

 Silene noctiflora roll themselves up inwards. 



2. Changes in position from increased turgidity. For the changes 

 under this condition the original paper must be consulted. Observa- 

 tions were made on the leaves of Chenopodiura album, Sulanum 

 tuberosum, Nicotiana latissima, Stellaria media, and Polygonum Con- 

 volvulus, and the petals of Convolvulus arvensis, Solanum tuberosum, 

 Silene noctiflora, and Calendula pluvialis and officinalis. 



The cases of those Papilionaceae of which the leaflets raise them- 

 selves erect in the evening, like Trifolium pratense and incarnatum and 

 Lupinus luteus, and of those which are depressed in the evening, like 

 Lupinus albus and Phaseolus, were subjected to careful investigation ; 

 and the mechanical causes of the various phenomena described are 

 minutely discussed in the paper. 



* ' Flora,' Isii. (1879) p. 11. 



3 D 2 



