STEMS. 17 



fow^ since gravitation influences the position which vegetables 

 present, as these beautiful experiments of Mr. Knight demonstrate, 

 a practical conclusion which seems to follow from the fact is this, 

 that the number of plants which may be placed upon a certain ex- 

 tent of soil, does not depend solely on the extent of surface ; and 

 that the power of production of a field which is very much sloped, 

 does not exceed its horizontal projection. With regard to creeping 

 plants, and with reference to meadows, it is clear that this principle 

 is not rigorously exact : but in so far as plants with isolated stems 

 are concerned, many agricultural philosophers, and among the num- 

 ber Davy,* have admitted it as perfectly indisputable. This opinion, 

 as M. Corrardf has judiciously observed, is founded on the geome- 

 trical jrinciple, which in itself is perfectly true, that an inclined 

 plane cannot be cut by a greater number of vertical perpendiculars 

 of a determinate thickness, than the horizontal plane which serves it 

 for a base. Thus, says Corrard, as buildings which rest on an in- 

 clined plane are raised perpendicularly to the horizon, it has been 

 concluded that an inclined plane can hold no larger an extent of 

 building than would the horizontal plane which it covers ; so that 

 inclinations of surface do not actually add to the extent of towns. It 

 is further a matter of absolute certainty, that as rain falls vertically, 

 the quantity of water collected upon the eaves of a house is precisely 

 the same as that which would be gauged in the same place upon a 

 horizontal surface, equal to that of the building. But we should 

 very much deceive ourselves, adds Corrard, if upon the same prin- 

 ciple we inferred that on the surface of an inclined plane we could 

 not have a tree more than upon the much smaller horizontal plane 

 which serves as its base. 



For although plants grow perpendicularly to the horizon, and may 

 in this respect be considered as so many vertical perpendiculars or 

 laminae, still, from circumstances which are peculiar to them, we 

 cannot here apply with propriety the geometrical principle in ques- 

 tion. Because, to make the application exact, it were necessary to 

 suppose that plants required no space around them to thrive, and that 

 the whole surface of the ground might be covered with their stems 

 without any space being left between them, and without this prox- 

 imity interfering with their growth and vigor. 



But such a supposition is impossible, inasmuch as it is absolutely 

 necessary that plants should have a certain amount of space, both in 

 the ground and in the atmosphere, in which to extend their roots 

 and stretch forth their branches. Supposing, therefore, the inclined 

 plane to be of considerably greater extent than the horizontal plane 

 which supports it, it will necessarily aflford to a larger number of 

 plants, the spaces which their roots require for their growth and 

 nourishment. In other words, upon the inclined surface there will 

 be a larger quantity of vegetable earth, and more of the nutritious 

 iuices favorable to vegetation ; and for these reasons the space which 



 Agricultural Chemistry, vol. i. 



t B. Corrard, Verhandel. von der Maatsch. te Haarlem, vol. xv. p. 308. 



2* 



