318 VEGETABLE IRRITABILITY. 



plants, especially in what are called Antheridia, bodies are met with 

 called Phytozoa (^[ 492), which also exhibit movements during a part 

 of their existence. 



658. Kemarkable movements have also been observed in the higher 

 classes of plants. The fovilla contained in the pollen-gram in a young 

 state, when moistened with water, exhibits movements when viewed 

 under the microscope. These movements have by some been referred 

 to irritability, but by Brown and other accurate observers, they are con- 

 sidered as merely molecular, and similar to what takes place between the 

 minute particles of inorganic matter as, for instance, finely powdered 

 Gamboge suspended in water. These fovilla movements are easily seen 

 in the very young pollen of Antirrhinum majus. Certain movements 

 also take place in the floral envelopes. Thus many flowers open and 

 close at particular periods (^[ 483 485) ; these phenomena depending 

 on light, temperature, and moisture. Leaves also, especially those 

 which are compound, are folded at certain periods in a distinct and 

 uniform manner. What was called by Linna3us the sleep of plants, is 

 the change produced on leaves by the absence of light. It is by no 

 means analagous to the sleep of animals. During darkness some are 

 slightly twisted and hang down ; others, such as pinnate and ternate 

 leaves, have the leaflets folded together, and frequently the common 

 petiole depressed. The youngest leaflets first exhibit these changes ; 

 and when the plants become old, and their tissues hardened, the 

 irritability is often much diminished, as is seen in Oxalises. The fold- 

 ing of the leaflets of compound leaves usually takes place from below 

 upwards, but sometimes in the reverse manner, as in Tephrosia 

 Caribsea ; so also with the common petiole, which is directed upwards 

 during sleep in the Cassias, and downwards in Amorpha. When be- 

 sides the common petiole there are partial petioles, as in the Sensitive 

 plant, they may be bent inwards towards each other, while the former 

 is bent downwards. 



659. Mimosa sensitiva and pudica, commonly called sensitive plants, 

 display these movements of their leaves in a remarkable degree, not 

 only under the influence of light and darkness, but also under mechani- 

 cal and other stimuli. They have bipinnate leaves with four partial 

 petioles proceeding from a common rachis, and each of the petioles is 

 furnished with numerous pairs (about twenty) of leaflets, which are 

 expanded horizontally during the day. During darkness, or when 

 touched or irritated in any way, each leaflet moves upwards towards 

 its fellow of the opposite side, which in its turn rises up, so that their 

 upper surfaces come into contact. When the movement commences 

 at the apex of the leaf, it usually proceeds downwards to the base, and 

 thence may be communicated to the leaflets of the next partial petiole, 

 and ultimately to the common petiole, which falls down towards the 

 stem. The partial petioles then converge towards each other, and 



