OF THE LEAVES. 737 



Marantaceae, Musacese, Aracese, Graminaceffl, &c., water is con- 

 stantly excreted in drops at certain periods of vegetation. 

 The most remarkable plant of this kind, however, is the Cala- 

 dium distillatorium, from which half a pint of fluid has been 

 noticed to drop away during a single night, from orifices placed 

 at the extremities of the leaves, and communicating freely with 

 internal passages. 



2. Absorption of Fluids by Leaves. — This matter has already 

 been considered when treating of the Functions of the Epi- 

 dermal Tissue and its appendages (see page 729), and need 

 not be further alluded to. 



3. Absorption and Exhalation of Gases by Leaves. — We have 

 already noticed (pp. 730 and 731) the property possessed by 

 the roots of absorbing liquid food from the medium in which 

 they grow, and also their supposed power of excretion (p. 732). 

 Whilst i^lants are thus intimately connected by their roots with 

 the soil or medium in which they are developed, they have also 

 important relations to the atmosphere by their leaves and other 

 external organs, which are constantly absorbing from, or exhal- 

 ing into it, certain gases. The gases which are thus absorbed and 

 exhaled by the leaves, have been proved by a vast number and 

 variety of experiments, to be essentially carbonic acid and oxygen. 

 Draper, Mulder, Cloez and Gratiolet, and others, also believe, that 

 leaves and other parts, exhale nitrogen when exposed to sun- 

 light. Again, the experiments of M. Ville would lead to the 

 conclusion, that plants under certain circumstances, also absorb 

 nitrogen from the air, but the recent able investigations of 

 Lawes, Gilbert, Daubeny, and Pugh, so far as they extend, do 

 not confirm his results, but tend, on the contrary, to negative 

 them. The whole matter connected with the subject of the ab- 

 sorption and exhalation of other gases rather than those of 

 carbonic acid and oxygen gases, is in an undetermined state, and 

 our future remarks will almost entirely apply to the latter. 



The absorption and exhalation of carbonic acid and oxygen 

 gases by the leaves, vary according to the circumstances in 

 which they are placed. Thus, when the green leaves of a 

 healthy plant are exposed to sunlight, all experiments show, 

 that carbonic acid gas is absorbed from the atmosphere, decom- 

 posed, leaving its carbon, which is the result of the decomposi- 

 tion, behind, and evolving its oxygen. It is in this way, that by 

 far the largest proportion of carbon, which, as will be presently 

 shown, is so essential to plants, is taken up by them. 



This evolution of oxygen by the green leaves and other green 

 organs may be readily observed taking place in the form of 

 bubbles, when a submersed aquatic pdant or some freshly gathered 

 leaves placed in water, are exposed to the direct rays of the sun. 

 These bubbles are oxygen gas. No such evolution of oxygen 

 takes place unless the water contains carbonic acid gas, and not 

 3^2 



