THE EPIDERMIS. 57 



liber, it follows from what has been stated, that a leaf is an 

 organ of which the upper system of veins is in communication 

 with the ascending, and the lower system with the descending, 

 current of sap. 



This statement must be understood to express nothing more than 

 what may be called the typical condition of a leaf, and especially of 

 such, as are thin and abundantly furnished with veins. In succulent 

 plants no layers can be distinguished, but the veins are dispersed among 

 pulp ; in membranous leaves it is uncertain whether more than one 

 layer is present ; finally in some there are three distinct layers, as in 

 Brexia spinosa, which has a middle system of coarsely and irregularly 

 netted veins, and immediately below both the upper and lower skin a 

 layer of much finer and closer oblique veins. 



A leaf has moreover a skin, or epidermis, drawn over it. 

 This epidermis is often separable, and is composed of an 

 infinite number ©f minute cells or cavities, originally filled with 

 fluid, but eventually dry and filled with air. In plants 

 growing naturally in damp or shady places it is very thin; 

 in others inhabiting hot, dry, exposed situations, it is hard 

 and thick; its texture varies between the two extremes, 

 according to the nature of the species. The epidermis is 

 pierced by numerous invisible pores, called stomates, through 

 which the plant breathes and perspires. Such stomates are 

 generally largest and most abundant in plants which inhabit 

 damp and shady places, and which are able to procure at all 

 times an abundance of liquid food ; they are fewest and least 

 active under the opposite conditions. It wiU be obvious, that, 

 in both these cases, the structure of a leaf is adapted to the 

 peculiar circumstances under which the plant to which it 

 belongs naturally grows. Now as this structure is capable of 

 being ascertained by actual inspection with a microscope, it 

 follows, as a necessary consequence, that the natural habits of 

 an unknown plant may be judged of with some certainty by a 

 microscopical examination of the structure of its epidermis. 

 The rule will evidently be, that plants with a thick epidermis, 

 and only a few small stomates, will be the inhabitants of 

 situations where the air is dry and the supply of liquid food 

 small ; while those with a thin epidermis, and a great number 



