274 



BOTANY 



In a vacuum, or on exposure to hydrogen and other gases — chloroform 

 vapour, etc. — movement also ceases, partly on account of insufficient 

 „ „ oxygen, and partly from the actual 



poisonous action of the gases them- 

 selves. If the state of rigour is not 

 continued too long, the original irrita- 

 bility will again return on the restora- 

 tion of normal conditions. 



The movements of irritability exhibited 

 by the staminal leaves of some Berberidaceae 

 (Berberis, Mahonia) and Compositae (Cynareae 

 and Liguliflorae) bear a certain relation to 

 those of foliage leaves. The bow - shaped 

 ' filaments of the stamens of the Compositae 

 straighten upon mechanical irritation. As 

 they frequently contract 10-20 per cent of 

 their length, the style becomes extended 

 beyond the anther - tube (Fig. 214). The 

 reduction in the length of the filaments is 

 accompanied by a moderate increase in their 

 jt a thickness, due to the elastic contraction of 



the cell walls, and the consequent expulsion 

 Fig. 214. -A single flower of Ccntaurm of water into the inteTOellulaI . spaoes , The 

 jacw. with perianth removed : A, sta- c „ , , , _ 7 . . 



mens in normal position ; B, stamens stamens of Berberis and Mahonm are only 

 contracted ; e, lower part of tubular sensitive to contact on the inner side near 

 perianth; s, stamens; u, anther-tube; their base, and as their contraction occurs 

 g, style ; P, pollen. (After Pfeffer, on ] y on the inner si( J e; the antners are t h us 

 en arge .) brought into contact with the stigma. Ex- 



amples of variation movements of carpellary leaves may be seen in the flowers of 

 Mimulus, Strobilanthes (Goldfussia), Martynia, Torenia, and other plants. The 

 two lobes of the styles of these flowers fold together when irritated. Similarly, in 

 the flowers of Stylidium, a sudden upward movement of the bent style occurs 

 when it is irritated by a touch ; but the style then loses its sensitiveness. 



VI. Reproduction 



The life of every plant is of limited duration. Death ensues, 

 sooner or later, and the decayed remains form a part of the surface 

 soil. All existing plants are descended from ancestral forms. A 

 spontaneous generation of new organisms from lifeless matter does 

 not, as far as experience teaches, take place, and all existing vegetable 

 life owes its existence to the capacity inherent in all organisms of 

 reproducing their kind. Reproduction is accordingly a vital power 

 which must be exercised by every existing plant species. In special 

 cases, it is true, abnormal forms, sports or monstrosities, are produced 

 unlike their parent plants ; but although they grow vigorously and 

 develop a strong vitality, they have lost the capability of giving rise 

 to equally strong descendants, or are unable to compete successfully 



