BOTANY. 457 



are commonly supported by a petiole or leaf-stalk, more or less 

 flexible, irritable in some species of Mimosa, and susceptible of 

 flexure in many leguminous plants. Many leaves, however, are 

 very thick and succulent, as those of the aloe ; others are hol- 

 low or tubular, as the onion ; some are subulate and pointed, 

 as in the pines ; and the same vegetable often bears different 

 descriptions of leaves at its root and stem. The leaves them- 

 selves are infinitely varied in point of form ; and many plants 

 bear compound leaves, or leaves of which one is formed by the 

 junction of several leaflets on a common leaf-stalk. Some aqua- 

 tic plants, as the Ranunculus aquatilis, have the leaves at the 

 surface of the water entire, while those below are divided into 

 filaments. The greater part of leguminous plants have pinnated 

 leaves ; in the Umbelliferce they are compound or laciniate ; in 

 the Rubiacece, verticillate ; opposite in the Labiatce, imbricat- 

 ed, sheathing, distichous, &c. To the base of the leaf-stalk are 

 often attached stipulce or bractece, small scales or floral leaves, 

 often of a different colour from the general foliage. 



The motion of many leaves and flowers, by which the blossoms 

 are closed in the absence of the sun, or at a particular time, accord- 

 ing to the species, has been termed the sleep of plants ; and this 

 is so regular in many that the period of the day may be indicated 

 from an observance of the flowers. Many of the Syngenesious 

 plants close their blossoms at the approach of rain ; and the 

 common daisy (Bellis perennis) is daily seen to expand and shut 

 its petals. To demonstrate that light is the principal agent 

 in regulating the opening of the blossoms, Decandolle placed, 

 -some plants in a darkened apartment during the day, and light- 

 ed it during the night by lamps. Some of the plants began 

 their daily sleep in the obscurity, and expanded at night ; but 

 others persisted in their natural predilections. Other flowers, 

 however, as the Convolvulus purpureus, open their blossoms 

 only at night. But it seems a general rule, that the appearance 

 and absence of light in the diurnal revolution of the globe regu- 

 lates the waking and sleeping season both of plants and animals. 



Many vegetables are furnished with spines, hairs, glands, and 

 scales, either for their protection from animals, or for security 

 against cold. Most alpine plants are covered by a down or hair 

 more or less long, to secure them from the inclemency of the 

 seasons. The spines are a prolongation of the ligneous part of 



