350 PRINCIPLES OF VEGETATION. 



principal caufe, viz. that power in the vefTels, whatever it is, that firft 

 puts the fap in motion before tranfpiration has commenced. 



Heat, moifture, and air, are the three chief circumftances that 

 promote the afcent of the fap. Hence nothing is more favourable to ve- 

 getation than warm weather, accompanied with rain ; on the other hand, 

 cold dry weather is its greateft enemy. In a wet cold feafon every thing 

 rots i and in hot dry weather every thing is parched. But the circum- 

 ftances moft favourable to vegetation are cloudy hot weather, inclina- 

 ble to thunder, fucceeded by plentiful rains. 



It would appear very improbable, that the fame matter could nourifh 

 fuch a variety of plants, differing fo effentially in fmell, tafte, figure, 

 &c. Much, however, may depend on'^the internal ftrufture and ar- 

 rangement of the veffels. If the veflels in any plant be uncommonly 

 fmall, parts will be rejected by that plant which would be abforbed by 

 one whofe veffels are larger. Nay, changes may be made in the crude 

 homogeneous nourilhment, by a fmall difference in the figure or adion 

 of the veffels. 



One plant will ftarve another by robbing it of its nourifliment, either 

 by extending its roots, and requiring a greater quantity of nourilhmenr, 

 or by abforbing the peculiar food which isneceffary for the growth of the 

 other plant. 



That a poifonous plant and its antidote will grow in the fame foil, 

 pnay be owing either to thefe plants imbibing different juices from the 

 earth, or to peculiarities in the ftrudture and aclion of their veffels. 



A ftalk of a lemon, grafted on a branch of an orange-tree, grew, ri- 

 pened its fruit, and preferved rhe figure and all the other qualities be- 

 longing to a lemon. Tiiis plainly indicates, that the organization of 

 the lemon had given a different modification to the juices of the orange, 

 through the intervention of which it received its nouriffiment. 



It is alfo certain, that the different parts of the fame plant have fre- 

 quently various fmells, taftes, &c. although the nourifliment derived 

 from the root muft be the fame. This is an evidence, that the different 

 ftrufflure of parts in the fame plants is capable of producing very fenfi- 

 bie changes in the nature and quality of the fap. 



Repeated experiments ffjow, that many plants of very oppofite qua,- 

 lities, and even trees, have been nourifhed and brought to maturity by 

 water alone. 



The roots of plants are fitted to abforb every fluid that comes 

 within their reach. They have been found by experiment to imbibe 



fluids 



