CAUSES OF THE CIRCULATION. 65 



In the imperfect plants which, having no vascular system, as the 

 lichens, mushrooms, etc., no sap ascends, as they imbibe fluids from 

 the air. 



The cause, of the circulation in opposition to gravitation, is not well 

 understood; it is, however, the apparent result of mechanical or 

 capillary action, promoted by heat. This is the popular opinion at 

 least ; but it fails in dead plants, as the sap will not rise in them on the 

 principles of capillary attraction. It will be seen, therefore, that 

 what we have said before on the circulation and functions of plants 

 generally should elicit further inquiry, and that we should not be con- 

 tent with the summary declarations of those predetermined to admit 

 no identity between the functions of plants and animals. Indeed, the 

 process is undoubtedly like that of the circulation in the animal system, 

 to which, throughout vegetable physiology, we observe a close func- 

 tional analogy, however much some may affect to discard the striking 

 resemblances. On arriving at the leaves the sap is exhaled in the 

 form of pure water, while the important substances it contained are 

 deposited in the leaf. The process of perspiration may be plainly 

 observed in the grasses and other plants. Thus about 2-3ds of the sap 

 absorbed by the roots is exhaled ; the remaining 3d containing the 

 nutritive properties distributed through it being retained. This would 

 appear like the chyle of the animal blood, and the leaves as peforming 

 the functions of lungs ; the difference being that plants in light inhale 

 carbonic acid and exhale oxygen, which is reversed in the dark ; while 

 animals inhale oxygen and exhale carbonic acid. The carbon of the 

 sap is further fitted for nourishment by the the absorption of oxygen 

 through the leaves during the night, to convert the carbon into carbonic 

 acid. The same process it does also by decomposing the water absorbed 

 by the roots, to obtain its oxygen. 



The cambium is the sap after its elaboration in the leaves, by which 

 it is rendered suitable for nourishment. It then descends through a 

 system of vessels between the liber, the yiternal layer of the bark, 

 and the young wood, or alburnum, contributing at the same time to 

 the formation of a new external layer of wood and internal layer of 

 bark, as well as the formation of new buds and roots. The descent 

 of the sap and the progress of these important functions are arrested 

 by cutting through the bark, when the plant dies. The proper juices 

 are all the fluids except the sap and cambium, and are oils, gums, etc. 

 They are, in fact, secreted by glands from the latter, as the many fluids, 

 such as tears, saliva, etc., are secreted from the blood by the glands of 

 the animal body. 



The epidermis, cellular integument and cortex constitute the bark. 



The first is also called the cuticle, as the scarf or outer skin of animals 



is called. It varies in thickness in plants, from the delicate covering 



of the rose-leaf to the ragged bark of the oak or walnut ; or like the 



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