14 VITAL ACTIONS. 



minute crevices, and to pass on from place to place 

 as fast as the food in contact with, them is consumed. 

 So that plants, although not locomotive like animals, 

 do perpetually shift their mouths in search of fresh 

 pasturage, although their bodies remain stationary. 



28. The only known exceptions to the rule that 

 roots do not lengthen by a general distension of their 

 tissue, occur in parts growing in air or water, which 

 are non-resisting media, or in certain endogenous 

 trees, whose roots lengthen to such a degree as to 

 hoist the trunk up into the air off the ground, with 

 which it at first was level. 



29. It is not, however, merely in length that the 

 root increases ; if such were the case, all roots would 

 be mere threads. They also augment in diameter, 

 simultaneously with the stem, and under the influence 

 of exactly the same causes. Neither is it by an em- 

 bryo alone that roots are formed. A plant, once in 

 a state of growth, has the power of producing roots 

 from various parts, especially from its stem, and from 

 older roots. 



30. The immediate cause of the formation of roots 

 is involved in obscurity, and is one of the most im- 

 portant parts of vegetable physiology still to be in- 

 vestigated with reference to horticulture. "We all 

 know how difficult it is to cause the cuttings of some 

 kinds of plants to produce young roots, and how ra- 

 pidly they are emitted by others ; it is to be supposed, 

 that the difficulty would be diminished in all such 

 cases, if we knew exactly under what circumstances 

 roots are formed. Nothing, however, sufficiently cer- 



