134 MASSACHUSETTS HORTICULTURAL SOCIETY. 



peculiar functions, which present questions of intense interest to the 

 student of nature. 



Anatomicalh' considered the leaf is a portion of the substance of 

 the bark, expanded into a broad, thin plate by means of a woody 

 framework, or skeleton, issuing from the inner part of the stem. 

 Tliis plate is called the lamina, or blade of the leaf, and consists of 

 two parts, the framework and the parenchyma. The framework is 

 made up from the branching vessels of the foot-stalks ; which are 

 woody tubes pervading the parenchyma, and carrying nourishment 

 to every part. From the analogy of their functions these vessels 

 are called veins. The parenchyma consists of two parts or strata, 

 more or less distinct ; arranged differently in different leaves accord- 

 ing as their natural position is horizontal or vertical. Externally 

 the leaf is covered with a layer of empty, united cells, mostly tubular, 

 forming a superficial membrane called the epidermis, which is 

 analogous to the cuticle that covers our own bodies. Its office in the 

 leaf is to check evaporation. The surface of the parenchyma, 

 immediately beneath the epidermis, is composed of one or 

 two layers of oblong cells, placed perpendicular!}' to that sur- 

 face, and more compact tlian the la3'er of cells beneath them, 

 which constitutes the lower stratum, and which contains in 

 addition, in common with the whole epidermis, the stomata, 

 or mouths, which are little clefts through the epidermis, and 

 are always placed over and communicate with the inter- 

 cellular passages. These little openings are guarded by valves, 

 which are supposed to regulate transpiration. The number of 

 these mouths, or stomata, is simply astonishing, A single square 

 inch of surface of the leaf of the common garden rhubarb contains 

 5,000, the garden iris 12,000, the pink 36,000, and the hydrangea 

 160,000. Note the immense number of them. We shall allude to 

 this point further on. Our leaf is also provided with glands, which 

 are cellular structures, serving to elaborate and hold the peculiar 

 secretions of the plant, such as aromatic oils, resins, hone}', 

 jioisons, etc. A gland may be merely an expanded cell at the 

 summit or base of a hair, or it may be a peculiar cell under the 

 epidermis, giving to the organ a punctate appearance, as in the 

 leaf of the lemon. Other glands are compound, and are either 

 external, as in the sundew, or internal reservoirs of secretions, as 

 in the rind of the orange. Whatever their construction or posi- 

 tion, the}' are important adjuncts to the legitimate office of the 

 leaf. 



