VEGETABLE lEEITABILITY. 375 



Prolifera, exhibit motions which depend on the presence of cellular 

 hair-like processes called cilia. These mobile 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. 467, p. 265) there are two of 

 these cilia or filiform tentacida, which project from a colourless 

 rostrum. In Chsetophora elegans, var. fusiformis, four have been 

 seen (fig. 468, p. 265) ; in Prolifera (fig. 469, p. 265) there is a circle 

 of cilia, and in Vaucheria (fig. 478, p., 269) the spore is entirely 

 covered with very short cUia, the vibration of which determines their 

 forward movement. These spores, from their movements, have re- 

 ceived the name of Zoospores (p. 265). Mr. Thwaites accounts for 

 the rhythmical movements of cilia by electrical currents. In certain 

 cells of Cryptogamic plants," especially in what are called Antheridia, 

 bodies are met with called Phytozoa or Spermatozoids (p. 265), which 

 also exhibit movements during a part of their existence. They are 

 well seen in (Edogonium (p. 271), Sphseroplea (p. 272), Saprolegnia 

 (p. 273), Fucus (p. 273), Hepaticse (p. 276), Mosses (p. 277), and 

 Ferns and their allies (pp. 279, 280). 



Eemarkable movements have also been observed in the higher 

 classes of plants. The foviUa contained in the poUen-grain in a young 

 state, when m,oistened 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 

 considered as merely molecular, and similar to what takes place be- 

 tween the minute particles of inorganic matter — as, for instance, finely 

 powdered Gamboge suspended ia water. These fovilla movements 

 are easily seen in the very young pollen of Antirrhinum majus. Cer- 

 tain movements also take place in the floral envelopes. Thus many 

 flowers open and close at particular . periods (p. 262); these pheno- 

 mena 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 Linnaeus the 

 sleep of plants is the change produced on leaves by the absence of 

 light. It is by no means analogous 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 are hardened, the irritability is often much diminished, as is seen 

 in Oxalises. The folding of the leafiets of compound leaves usually 

 takes place from below upwards, but sometimes in the reverse manner, 

 as in Tephrosia Oaribsea ; so also with the common petiole, which is 

 directed upwards during sleep in the Cassias and downwards in 

 Amorpha. When,' besides the common petiole, there are partial 



