LEAVES. 71 



with pores* for exhaling and inhaling gases ; and as they pre- 

 sent to the air a more extended surface than all other parts of 

 a plant, they are of great utility to the vegetable, by imbibing 

 suitable nourishment, and throwing off such gases as would be 

 useless or injurious. 



We have seen how the bud is formed, and by what curious 

 means the principle of vegetable life which it contains is pre- 

 served and protected through the cold and dampness of winter. 

 In the spring, when the sun has turned his course towards the 

 north, recrossed the equator, and is advancing towards the tro- 

 pic of Cancer ; the vegetable world, quickened by its influence, 

 begins to awaken from its dormant state ; the leaf buds expand, 

 and soon bursting their envelopes, the green leaves come forth. 

 The manner in which the leaf, before it expands, lies wrapped 

 up in the scales of the bud, is called Foliation ; this presents an 

 interesting study, and is said to be sufficiently various, in differ- 

 ent families of plants, to afford a mark of distinction between 

 them. 



In the buds of grasses the leaves are said to be rolled to- 

 gether ; in the rose, one side of the leaf is doubled upon the 

 stem ; in the maple, the leaves are plaited or folded up like a 

 fan ; in the blue bell, imbricated, or laid over one another like 

 tiles on the roof of a house ; in the monk's hood they are bent 

 downwards towards the petiole. 



Some plants are destitute of leaves; they are then called 

 Aphyllous, which term signifies wanting leaves. 



In determining the species of plants, the leaves are much re- 

 garded ; specific names are often given from some circumstance 

 of the leaf, for example ; the HEPATICA tritoba is that species 

 of the Hepatica, which has leaves with three divisions called 

 lobes ; the VIOLA rotundifolia is a species of violet with round 

 leaves. 



A knowledge of the various appearances presented by leaves, 

 is of great importance; in order to become acquainted 

 with all their varieties, considerable practice in the analysis of 

 plants is necessary. Engravings will assist you in understand- 

 ing the definitions, but you must chiefly consult nature. There 

 are many terms to express the varieties observable in leaves ; 

 we shall here explain some of the most important. 



* Technically called stomas. 



Utility of leaves to the whole plant The period at which loaves appear 

 Foliation Aphyllous leaves Leaves furnish specific characters. 



