130 FLOWER-GAKDENING. 



substances are as fatal to tlie species tliat secrete tliem as to 

 auy other species. 



36. Hence whatever is known to produce any specific dele- 

 terious action upon leaves or stems, such as certain gases (298) 

 and mineral or vegetable poisons, will produce a much more 

 fatal effect upon the spongioles. 



37. These organs have no power of selecting their food, but 

 vrill absorb whatever the earth or air may contain, w^hich is 

 sufficiently fluid to pass through the sides of their tissue. 



38. So that if the spongioles are developed in a medium 

 which is of an unsuitable nature, as they will still continue to 

 absorb, they cannot fail to introduce matter which will prove 

 either injurious or fatal to life, according to its intensity. 



39. This may often explain why trees suddenly become un- 

 healthy, without any external apparent cause. 



40. Plants have the power of replacing spongioles by the 

 formation of new ones ; so that an individual is not destroyed 

 by their loss. 



41. But this powder depends upon the cooperation of the 

 atmosphere, and upon the special vital powers of the species. 



42. If the atmosphere is so humid as to hinder evaporation, 

 spongioles will have time to form anew ; but if the atmosphere 

 is dry, the loss by evaporation will be so much greater than 

 can be supplied by the injured roots, that the whole system 

 will be emptied of fluid before the new spongioles can form. 



43. This is the key to Transplantation. 



44. As roots are destitute of leaf-buds, and as leaf-buds are 

 essential to the multiplication of an individual (108), it should 

 follow that roots can never be employed for the purpose of 

 multiplication. 



45. Nevertheless, roots when woody have, occasionally, the 

 power of generating adventitious leaf-buds (IV.) ; and when 

 this is the case, they may be employed for the purpose of mul- 

 tiplication ; as those of Cydonia Japonica, etc. 



46. The cause of this power existing in some species, and 

 not in others, is unknown. 



