STRUCTURE OF THE CUTICLE. 71 



be scarcely separated from the tissue around; hence it was. long 

 supposed that the nutritious sap, or proper juice as it is gene- 

 rally termed, flowed in mere spaces amongst other tissues, 

 and not in distinct tubes. T.he existence of these, however, is 

 now well established ; and there can be little doubt that, like 

 the straight ducts, they take their origin from cells, the partitions 

 between which are broken down, so as to form a complete net- 

 work of canals. 



88. In future chapters, the combinations of these tissues in 

 the several organs, such as the Stem, the Leaves, the Flowers, 

 &c. will be described ; but it may be well here to speak of one 

 peculiar modification of cellular tissue which is seen in all these 

 parts, that, namely, which forms the cuticle or skin in which 

 they are enveloped. The existence of this is easily shown in 

 many leaves without preparation. From the leaf of the common 

 garden Iris, for example, it may be easily stripped, or from the 

 under side of that of the London Pride ; and from every leaf it 

 may be easily removed, after being soaked for a few days in 

 water. This cuticle is found to be usually transparent and nearly 

 colourless. If when separated it should appear coloured, this is 

 due to the adhesion to it of some of the cellules of the fleshy 

 portion (or parenchyma) of the leaf ; these will afford an oppor- 

 tunity of examining the form and structure of these cellules ; and 

 they may then be wiped away, leaving the membrane perfectly 

 smooth and colourless on both sides. Now when this is examined 

 with a sufficient magnifying power, it is seen to consist of a 

 number of flattened cells in close contact with each other ; and 

 these cells contain either air, or a colourless fluid. Their form is 

 very different according to the kind of plant examined. Some- 

 times they are of a regular oblong and their sides straight ; whilst 

 in other instances they are of very irregular form, and lock into 

 one another like the pieces of a dissected map. 



89. Though the cuticle usually consists but of one layer of 

 cells, it sometimes contains two or even three, especially in plants 

 naturally growing in warm climates ; and in the Oleander four 

 may sometimes be distinguished. Its office appears to be, to 

 prevent the moisture of the soft succulent tissues beneath from 



