302 THE DISEASES OF PLANTS. 



Emily. That is very evident in the superior vegeta- 

 tion of a single tree, which has ample space for its branch- 

 es, and food for its roots, to that of a tree in a crowded 

 forest, where every inch of ground is disputed by sur- 

 rounding plants. 



Mrs. B. But, independently of this general competition 

 for food, there are various other modes by which some 

 classes of plants are noxious to others. Among these the 

 parasitical plants stand pre-eminent. There are two 

 classes of this description, distinguished by the epithets 

 of false and true. The false parasite fixes itself to the 

 plant, without feeding on its juices; while the true para- 

 site feeds on the plant to which it adheres. These two 

 classes are each subdivided into external and internal 

 parasites, denoting the parts of the plant which they attack. 



The false parasites consist of mosses, lichens, and fun- 

 gi, which grow on living plants just as they would grow 

 on a rock or a dead tree. 



Emily. Such as the various mosses which grow on 

 the stems of fruit-trees. But, if they do not feed on the 

 tree, whence do they derive their nourishment ? 



Mrs. B. From the moisture of the atmosphere, and, 

 possibly, from the relics of some preceding mosses, which 

 supply a few particles of vegetable mould. 



Caroline. If they do not feed on the juices of the 

 tree, in what manner do they injure it ? 



Mrs. B. Chiefly by attracting moisture to the stem, 

 and thereby endangering the wood ; and also by afford- 

 ing a lodgment for insects. In these temperate climates, 

 however, the harm they do is not of a very serious na- 

 ture ; but, in tropical regions, parasitical plants grow 

 with such luxuriance (the vanilla, for instance,) that the 

 tree suffers mechanically from the weight of the mass it 

 has to bear. 



Emily. I recollect, in the Caschines of Florence, 

 seeing many of the elm-trees so completely covered with 

 ivy, that I at first sight concluded the tree itself was of 

 that description. 



Mrs. B. Ivy is a creeping plant, not a parasite. Its 



1627. What is the difference between false and true parasites'? 

 1628. Into what other classes are parasites divided! 1629. Of what 

 do false parasites consist! 1630. From what do they derive nourish- 

 ment, if not from the tree on which they grow! 1631. If they do not 

 feed on the tree, how do they injure it! 1632. How do trees suffer 

 from parasites in tropical regions! 



