304 THE DISEASES OF PLANTS. 



Caroline. It grows, then,' in the light and air, which 

 must be an equally uncongenial soil ! 



Mrs. B. True ; and it no sooner makes this discovery, 

 than it changes its course, and, curving round, retraces 

 its steps towards the branch whence it sprouted. 



Caroline. Just as if it were conscious that the soil it 

 had abandoned, was that in which it was destined to grow. 



Mrs. B. It is said that it is in order to avoid the light, 

 that it alters its course ; for roots, you know, dread the 

 light as much as leaves and branches delight in it. 



Emily. The dread of the former may, no doubt, be as 

 mechanically explained as the delight of the latter. 



Mrs. B. Certainly. The extremity of the root having 

 now acquired sufficient strength, as soon as it comes in 

 contact with the branch, pierces the bark, and plants it- 

 self in the alburnum, whence it sucks up its food, just as 

 another plant would do from the soil. 



Caroline. With the advantage that its food is already 

 prepared ; it can therefore scarcely require leaves to con- 

 vert the sap into cambium. 



Mrs. B. I beg your pardon. The soil from which it 

 feeds is the wood, not the bark ; it is therefore the rising, 

 not the descending sap which it receives ; the rnisletoe, 

 and the tree to which it adheres, may therefore be consid- 

 ered as the same individual plant. The parasite receives 

 the sap after the same manner as the branches of the tree, 

 and like them, requires leaves for its elaboration. 



Emily. This junction is very analogous to a natural 

 graft. 



Mrs. B. On the contrary, it is quite the reverse. In 

 a graft, it is the vessels of the liber which unite ; whilst 

 the misletoe strikes its little root through the bark into the 

 wood, and the junction of the vessels takes place in the 

 alburnum. 



Emily. Is it not wonderful that so young and tender 

 a root should be able not only to pierce the bark, but even 

 to penetrate the wood ? 



Mrs. B. It is, indeed ; but observe that it does not 



1638. What takes place, when th,e light and air are found prejudicial 1 ? 

 1639. How does the light effect the roots of plants! 1640. What 

 takes place, when the extremity of the roots have acquired sufficient 

 strength 1 ? 1641. Why does Caroline suppose it does not need leaves'? 

 1642. How does Mrs. B. show that she is mistaken in this particu- 

 lar! 1643. Why may not the misletoe be viewed as a graft upon the 

 treel 



