CH. II.] THE ROOT. 105 



veutm-e to alight upon tliem : no sooner do their legs 

 touch these parts than the sides of the leaves spring 

 up, and loclving their rows of prickles together, 

 squeeze the insects to death. The well known 

 sensitive plant (Mimosa sensitivaj shrinks from the 

 shghtest touch. Oxalis sensitiva and Smithia serisi- 

 tiva are similarly irritable, as are the filaments of 

 the stamens of tbe berbeny. One of this irritable 

 tribe, Hedysaruni gyrans, has a spontaneous motion ; 

 its leaves are frequently mo^'ing in various dh'ections, 

 without order or co-operation. When an insect 

 inserts its proboscis between the converging anthers 

 of a kind of dog's bane ( Apocynum androsscemifoliwnj 

 they close with a power usually sufficient to detain 

 the intiiider mitil death. The more I study the 

 phenomena of vegetation the more I feel convinced 

 on this point. How often have I heard a farmer 

 reply to an obseiTation upon the tardy growth of 

 turnips, " They will not grow apace, until their 

 leaves are large enough for the wind to take hold of 

 them;" and this is only because plants cannot be 

 healthy and vigorous ^rithout exercise. Mr. Ivnight 

 found that trees which were regularly shaken every 

 day in his green-house, grew more rapidly and were 

 stronger than others which were kept still. 



The stimulating powers of excrementitious manures 

 arise from the salts of ammonia they contain. Sir 

 H. Da\-y foimd vegetation assisted by solutions of 



