806 PHYSIOLOGY. 



1. Movements which evidently depend on external influences. 



These are divided into two : — 

 a. Periodical. 

 h. Not periodical. 



2, Movements independent, at least to some extent, of ex- 



ternal influences, which are also divided into two : — 

 a. Periodical. 

 h. Not periodical. 



1. Movements depending on external Influences. — a. 

 Periodical. — Under this head we include such movements as 

 those of certain leaves and the petals of flowers, which occur 

 at particular hours, the organs remaining in the new position 

 thus taken up until the return of a particular period, when they 

 resume as nearly as possible their original position. In leaves, 

 these periodical movements consist in the closing up of such 

 organs towards the evening and their expansion in the morning. 

 In the petals of flowers great differences occur in opening 

 or closing at particular hours of the day ; and, by observing 

 these changes in a variety of flowers, Linnseus and others have 

 drawn up what has been termed a floral clock. This periodical 

 closing up of leaves and flowers has been called the sleep of 

 plants. The compound leaves of certain Leguminosse and 

 Oxalidacese are marked illustrations of these periodical move- 

 ments, which are probably all indirectly dependent upon the 

 varying conditions of light to which the parts of the plant in 

 which they occur are exposed. 



h. Not 'periodical. — Such movements are exhibited in a num- 

 ber of plants both in the leaves and in their reproductive organs. 

 In the leaves they are well seen in certain species of Oxalis, 

 Mimosa {fig. 347), in LioncBa muscipida {fig. 349), &c. In the 

 Eeproductive Organs they may be noticed in the curving inwards 

 or outwards of the stamens of certain plants, such as those of 

 Berberis vulgaris and other species, Parietaria judaica, Helian- 

 themuni vidgare and other Cistacese ; also in the stigmas of the 

 Lobeliacese, and in the style of Goldfitssia aiiisophylla, ^-c. All 

 the above movements are produced by external agency, such as 

 the action of insects, the agitation caused by the wind, &c. 



2. Movements independent, at least to some extent, of 

 external Influences. — a. Periodical. — These movements are 

 seen in the leaflets of certain tropical species of Desmodium 

 (Hedi/sarum), and more especially in those of Desmodium 

 gyrans {fig. 1127). The leaf in this plant is compound, and 

 bears three leaflets. There are also two other rudimentary 

 leaflets, also marked b, near the terminal one ; the terminal 

 one, a, being much larger than the two lateral ones, b. The 

 large terminal leaflet, a, when exposed to the influence 

 of a bright light, becomes more or less horizontal, but it 



