VEGETABLE IRRITABILITY. 317 



ricula, and the Carnation. Some flowers, with spurred petals in their 

 usual state, as Columbine, are changed so that the spurs disappear ; 

 and others, as Linaria, in which one petal only is usually spurred, are 

 altered so as to have all the petals spurred. 



SECTION IV. SOME GENERAL PHENOMENA CONNECTED ; 

 WITH VEGETATION. 



1 . VEGETABLE IRRITABILITY. 



656. Under this head are included certain sensible movements of 

 living plants, exercised without any direct application of mechanical 

 force, and not referable to mere elasticity, or the hygroscopic nature 

 of the tissues. These motions are influenced chiefly by light and heat, 

 and, like many phenomena occurring in organized beings, they cannot 

 at present be explained by chemical or mere mechanical laws. They 

 may, however, be excited by stimuli of a chemical or mechanical 

 nature. Although the cause of them is obscure, still, in some instances, 

 their use is obvious. 



657. Among the lowest classes of plants there are some peculiar 

 movements of this kind. The simplest members of the Sea- weed tribe 

 occasionally move throughout their whole substance. Oscillatorias, 

 which are filaments composed of cells placed end to end, containing 

 fluid and granular matter, have an undulating movement, by means of 

 which they advance. When placed in fluids under the field of the 

 microscope, some of them may thus be seen to pass from one side to 

 the other. The filaments sometimes twist up in a spiral manner, and 

 then project themselves forward by straightening again. The motions 

 are influenced by temperature and light, and by some are considered as 

 being connected Avith the production of new cells. The spores of many 

 Cryptogamic plants, especially species of Vaucheria, Conferva, and Proli- 

 fera, exhibit motions which, according to Thuret and Decaisne, depend 

 on the presence of cellular hair-like processes, called cilia or tentacula. 

 These motive organs are in a state of constant agitation, which lasts for 

 some hours, becoming slower, and finally ceasing after germination has 

 commenced. In the spores of Conferva glomerata and rivularis (fig. 

 431), there are two of these cilia or filiform tentacula, which project 

 from a colourless rostrum. In Chajtophora elegans, var. fusiformis, 

 four have been seen (fig. 432) ; in Prolifera (fig. 433), there is a circle 

 of cilia, and in Vaucheria (fig. 434), the spore is entirely covered with 

 very short cilia, the vibration of which determines their forward move- 

 ment. These spores, from their movements, have received the name of 

 Zoospores (^[ 492). Mr. Thwaites accounts for the rhythmical move- 

 ments of cilia by electrical currents. In certain cells of Cryptogamic 



