FUNCTIONS OF PLANTS. 33 



the natural habitations of plants, but with their natures. In all, whether 

 thick or thin, it is pierced with numerous pores, called stomata, which can- 

 not be seen with the naked eye, but through which the leaf inhales and 

 exhales gases, and perhaps watery matters. The stomata are generally 

 largest and most abundant in aquatic or marsh plants, or plants adapted by 

 nature for shady places, and which can procure at all times an ample supply 

 of liquid food ; and they are, on the contrary, fewest and least active, in 

 warm, open, airy situations, where liquid food is less abundant. Thus it 

 appears that the structure of a leaf being adapted to the particular situation 

 in which the plant naturally grows, it may serve to indicate what sort of 

 culture may be most suitable for plants of which we have previously known 

 but little. It is evident, however, that this criterion must be of rather 

 difficult application in practice, excepting by gardeners who are scientific 

 botanists, and have been in the habit of using powerful microscopes. 



123. There are some plants which produce no leaves, or in which the 

 leaves are so small, and drop off so soon after they are formed as to leave 

 no traces of them on the bark. Instances of this kind are found in the 

 genera Cactus, Epiphyllum, Opuntia, Stapelia, and even, but in a much 

 less degree, in some species of Asparagus, Spartium, and Ge'nista. In 

 all such cases, the functions that are in other plants, performed by the 

 leaves, are performed in these plants by the bark. The functions of the 

 leaves, and of the green parts of the bark, and of the plant in general, 

 are to absorb carbonic acid, and with the aid of light and moisture, to appro- 

 priate its carbon. Carbonic acid may enter the plant by the roots, by the 

 leaf^ and by the green parts of its bark. When either of these parts is 

 exposed to the action of the sun, the carbonic acid is decomposed, oxygen is 

 given off, and the carbon is fixed in the leaf or bark. The escape of the 

 oxygen may be proved by immersing a leaf in water, and exposing it to the 

 sun. If a leaf be immersed in water in the shade, little or no air will be 

 given off, and that little will be found to be carbonic acid gas. Plants, it has 

 been found, decompose carbonic acid during the action of solar light on the 

 leaves during the day, and form it again in the shade and during night ; 

 and hence, in a healthy plant, the decomposition of carbonic acid and the 

 liberation of oxygen during the da}', and the absorption of oxygen and the 

 liberation of carbonic acid gas during the night, are perpetually going on 

 while the plant has leaves, or is in a growing state. The healthiness of a plant, 

 other circumstances being alike, is in proportion to the quantity of carbonic 

 acid decomposed during the day, and this will depend on the quantity of 

 light it receives during the same period. Plants which naturally grow in 

 shady situations form exceptions to this general principle ; probably, because 

 the powerful action of the sun on their leaves would cause them to perspire 

 water in too great abundance. 



124. In conclusion, it may be observed, that all the matters assimilated 

 by plants, whether of a general kind, such as carbon, or of a specific nature, 

 such as acids and alkalies, resins, oils, &c., are effected by the action of 

 light on the leaves ; and hence, as we have said before (9), the treatment 

 of the leaves of plants is of far greater importance than the treatment of any 

 other part whatever. 



125. The action of the leaf generally ceases when the part of the stem to 

 which it is attached is matured, or when the fruit which is nearest to it is 

 ripened. At that period the leaf commonly changes colour, ceases to decom- 



