Plants gt'owing on their superincumbent Soils. 419 



selves from place to place, so various and complete as to form 

 one of the chief objects of interest in the whole study of natu- 

 ral history. In the wide world there is no lack of room for 

 its vegetable inhabitants, no lack of places which, to our views, 

 might be benefited by a supply of vegetation 5 but each por- 

 tion of matter has its limits, its place, beyond which it cannot 

 proceed. Thus, then, many of the vegetable population 

 perish, as it were, by the evils of emigration, in an unsuitable, 

 a foreign land ; and thus the accidental occurrence of a rare 

 plant in a new and unexpected place leads, erroneously, to 

 the conclusion that it is native there. 



There are many plants purely of vegetable origin, which 

 may almost be denominated parasite. Woodroof and wood- 

 sorrel, for example, are met with every day feeding on the 

 leaves of trees which are only converted into mould. The 

 universality of vegetable soil will produce abundance of ano- 

 malies ; but they are exceptions, and scarcely bear, either in 

 variety or in nature, on the question at issue. 



3. There are also many which are purely parasitical, 

 i. e. feed on living plants. Of this nature are the well-known 

 mistletoe, the dodders, and broom rapes, with numerous mosses, 

 lichens, and fungi. Of the fungi, a most extensive class are 

 known to subsist on leaves and stems of certain genera, or 

 their species, or even particular varieties. This last is an un- 

 travejled field in botany, touched on by Dr. Greville of Edin- 

 burgh in his late work, and ably illustrated by a Hortus Siccus, 

 with descriptions, the work of an ingenious and modest man, 

 the curator of the physic garden at Oxford. 



4. Or lastly, these anomalies are few, and not plentiful, 

 where they occur. Now, I contend, that the proper habitat 

 of a plant is where it is found growing freely and constantly, 



Manchester, October, 1829. 



