,ft BOTANY OP THE LIVING PLANT 



I ,. ^ ,„ this plant explodes with a report Like a P^;**. *£ 

 throws the h isoul to a distance oi many yards (Fig. 102). These and 



other explosive frnita will be considered again on p. 324. Mention may also be 

 milll . ()f th , hygroscopic awns of the grass Stipa. through the twisting move- 

 meats of whii h the seed tends to be pushed into the soil ; and also of the move- 

 ments of the peristome teeth of Mosses. An example of a rather different type 

 of hygroscopic movement is seen in the dehiscence of the Fern sporangium, 

 which depends on changes in volume of the water present in the cavities 



Fig. 102. 



Whole fruit of Hura crepitans, before rupture of its woody carpels. (After Le Maout.) 



a, b, single carpels after the explosion, showing each coccus with gaping halves. The 



rupture happens suddenly, each coccus taking a wider shape ; the cocci and seeds 



are thus thrown asunder. c = a single large seed. The tree is native in Tropical America. 



of specialised cells of the annulus (Fig. 103). When exposed to a dry atmo- 

 sphere these cells gradually lose the water with which the cell-cavities are filled, 

 diminishing mass of water within each cell retains its continuity because of 

 the cohesive properties of water, and also continues to adhere to the cell walls : 

 the latter therefore suffer a deformation and the annulus as a whole curves back- 

 Fig. 103, A , C) . A point is ultimately reached at which the continuity of 

 the water within the cells is broken. The cells immediately resume their normal 

 shape and the annulus recovers with a sudden jerk (Fig. 103, B, D), throwing out 

 the spores. The rolling and unrolling of certain Grass leaves (p. 188) is also 

 due to changes in volume of the water present in special cells. 



It is evident from what has been said in the latter part of this Chapter 

 that the several organs of the plant exhibit a wide variety of move- 

 ments and curvatures. The plant as a whole is. however, fixed in 

 ition, a feature of difference from the animal which is to be 



