ANATOMY AND PHYSIOLOGY OF PLANTS. 235 



22. The epidermis is merely an extension of the gene- 

 ral covering of plants, though unquestionably, more or 

 less delicate in comparison to the epidermis of the stem 

 and other parts. 



If a leaf is taken and torn asunder, either in a transverse or longi- 

 tudinal direction, fragments of the epidermis, a fine and transparent 

 pellicle, will be seen projecting from the edge of the torn part. 



23. The parenchyma is a green and pulpy substance 

 constituting for the most part, the mass of the leaf, in- 

 terspersed with the .fibres or vessels of the leaf. 



24. The vessels of the leaf are the prolongations of 

 the petiole, divided into an immense number of ramifi- 

 cations, mutually embracing and intersecting one another, 

 and forming a sort of fabric similar to a piece of fine net- 

 work. This net-work, however, is double. 



The upper set of vessels corresponding to the upper surface of the 

 leaf, are considered as air vessels, through which the perspirable 

 matter is discharged ; whereas, the lower or under set, are imagined 

 to receive, prepare, and convey, the moisture imbibed from the 

 rising vapours of the earth, throughout the whole plant. 



25. There have been innumerable opinions advanced 

 to explain the use or functions of leaves ; but whatever 

 may be their precise intention in the vegetable economy, 

 it is generally agreed among botanists, that a different 

 office belongs to their upper and under surface, as hinted 

 in the last note. 



5JC. The upper surface is usually admitted as the 

 organ of respiration. 



