390 Arialogy between Vegetables and Animals 



each other, but in some cases they do not appear to do so< 

 Their shape is extremely irregular, although they are said to 

 be hexagonal. Some of them are thought to contain air, 

 others contain the oily and resinous secretions, but by far 

 the greater number contain a watery fluid. 



Now, dissection demonstrates to us, that a cellular tissue 

 exists very abundantly in every part of the animal frame ; 

 and in its character and uses it bears the strongest analogy 

 to that which I have just described, as being diffused through- 

 out the vegetable structure. We see it, for instance, sur- 

 rounding every organ of the body, and uniting together their 

 several parts. Its cells, according to the best anatomists, are 

 of an irregular shape. Most of them contain an albuminous 

 fluid, and others are filled with a peculiar oily matter, termed 

 fat. The former of these communicate with each other ; but 

 the latter have no such communication. The cellular tissue 

 of animals is composed entirely of membrane ; and, like this 

 substance, it exhibits no vital properties, and is therefore 

 neither contractile nor sensible. Besides the cellular tissue, 

 or membrane, as it is sometimes called, animal bodies pos- 

 sess other kinds of membrane ; the chief of which are, the 

 serous, the mucous, and the fibrous. Although there is 

 nothing at all similar to these in plants, so far as I know, yet 

 they are all, according to Haller, composed of condensed 

 cellular substance, which, as I have just shown, is equally 

 abundant in vegetables as in animals. The integument, 

 however, or outer covering, of plants, bears the strongest 

 similitude to the skin of animals ; which by the most eminent 

 physiologists is considered as one of the principal membranes 

 of the body. The outer investment, or skin, if it may be so 

 termed, of vegetables consists of three distinct layers. The 

 most external of these is called the epidermis ; the second is 

 called the cellular integument ; and the third the true bark. 



The epidermis is a thin membrane, which is spread over 

 the surface of every part of the plant. It covers the roots, the 

 stem, the branches, the flowers, and even the fruit ; and it 

 enters into and even lines the pores which exist in the leaves, 

 the stem, and other parts. It is formed only of a single layer. 

 It is generally so transparent that the colour of the parts be^ 

 neath it can easily be perceived. In some cases, however, it 

 is somewhat opaque, and occasionally it exhibits a certain 

 degree of colour. Although it gives free transmission to 

 moisture, vapour being not only exhaled but absorbed through 

 it, yet no pores have been detected passing through its sub- 

 stance. Its thickness varies, not only in different plants, but 

 in different parts of the same plant. In the fir, it is hard 



