Hints on Vegetation. 9 



trines however, ought not to be carried too far *, for air, however im- 

 portant, is not alone sufficient for the purposes of perfect vegetation. 

 On this subject, Sir Humphry Davy's opinion seems to be correct, That 

 " all the varieties of substances found in plants, are produced from the 

 " 8 ap ; that the sap is derived from the fluids in the soil, and it is 

 " altered by, or combined with principles derived from the atmos- 

 " phere." In regard to the influence of air on vegetation, a most in- 

 telligent physiologist observes, " That both in plants and animals, 

 " oxygen gas is uniformly converted into carbonic acid, during the 

 " exercise of the respiratory function ; and that by this chemical 

 " change in the air, the latent or specific caloric is set free, and en- 

 41 ters into the vegetable and animal systems-^" It is thus chiefly, 

 that the human frame, acquires heat internally. 



3. Water. That water is essential to vegetation, seems to be uni- 

 versally admitted. Indeed the luxuriancy of the growth of plants, 

 connected with the presence of moisture, led Van Helmont and others 

 to entertain the opinion, that water is the real and efficient source 

 of vegetation, and that all the various products of plants are gene- 

 rated by water alone. This opinion is erroneous, for no single sub- 

 stance affords exclusively food to vegetables. Earth, air and water, 

 are all necessary for that purpose. " The earth" is the laboratory in 

 which the food is prepared, and retained for use : The germination 

 or growth of the seed of plants does not take place, without the presence 

 of that valuable substance which " the air' contains, namely, oxygen 

 gas ; and the beneficial products to be found in putrescent manures, 

 cannot be taken up by the roots of plants, without the assistance of 

 " Water" Thus they all separately and respectively contribute, in 

 different ways, to promote the process of vegetation. 



Water is likewise of importance, as supplying the principal material 

 of the sap, which may be called the blood of plants, without which they 

 would perish, many instances being known of trees dying, when their 

 sap was exhausted. The great improvements which are made by the 

 mere watering of land, likewise prove the powerful effects of water. But 

 it seems more favourable to the growth of grasses than of grain, for 

 though, by irrigation, perpetual crops of grass could be obtained, yet, 

 experience has proved, that land, if cropped with grain, was completely 

 exhausted, though regularly watered, which could not have been the case 

 if water was the sole food of plants J. It is certain, however, that a large 

 proportion of every plant consists of water, and that water is the ve- 



4 



* Dr Bell thus proved the importance of air to vegetation. la the winter 

 season, he covered several young trees with varnish, and at the same time 

 wrapt them in wax cloth, leaving the tops of the branches only exposed to air. 

 They remained in this situation during the whole summer, when some of them 

 lived, though in a languid state, and put out a few leaves ; but those from 

 which the air had been more accurately excluded died, without a single excep- 

 tion. Manchester Memoirs, vol. ii. p. 421. 



f See Ellis on Air, part ii. printed an. 1811, p. 574 ; also, his very valuable 

 work, on Vegetable Physiology, in the Supplement to the 5th edition of the 

 Encyclopaedia Britannica. 



f Some philosophic experiments, as those of Dr Woodward, prove, that plants 

 will grow, with the application of pure rain water alone ; but the question is, 

 will rain water alone enable them to produce seed in perfection ? 



