OF LEAVES. 



49 



a compound substance, consisting of chlorine and sodium. In the 

 first definition, given without any reference to scientific principles, 

 there is nothing so definite as to afford a certain mark of distinction 

 between salt and other substances; in the chemical definition, we 

 have a test for salt, in a knowledge of its composition, which dis- 

 tinguishes it from all other substances. 



In botanical definitions, we do not include the constituent elements 

 of the vegetable substance; this belongs to the department of chem- 

 istry, but we consider the external forms and uses of the various 

 parts of the plant. 



The leaf is an expansion of the fibres of the bark, connected by a 

 substance, called the cellular tissue ; the whole is covered with a 

 green coat, or skin, called the cuticle. Leaves are furnished with 

 pores called stomas, for exhaling and inhaling gases. They present 

 to the air a more extended surface than all the other vegetable or- 

 gans, and are of great importance by imbibing suitable nourishment, 

 and throwing oft" such gases as would be useless or injurious to the 

 plant. 



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

 principle of Ufe which it contains, is protected through the cold and 

 dampness of winter. In the spring, when the sun, having recrossed 

 the equator, is advancing towards our hemisphere, the vegetable 

 world,^quickened by its influence, begins to awaken from a dormant 

 state ; the buds expand, and bursting their envelopes, the new 

 branches, bearing leaves and flowers, come forth. 



The manner in which the leaf 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 different families of plants, to afford 

 a mark of distinction between them. 

 Fi 



. 37. 



Figure 37, at a, shows a 

 young leaf of the currant; 

 this is folded. At b, is a 

 young leaf of the Aconi- 

 tum, "(monk's-hood ;) this 

 is inflected. At c, is the 

 young leafof a fern, (aspi 

 dimiu) this is circinate, or 

 rolled from the summit to- 

 wards the base. 



Some plants are desti- 

 tute of leaves; they are 

 then called Aphyllous, from 

 the Greek, a, to want, phyl- 

 lon, a leaf 



In determining the species of plants, the leaves are much regarded. 

 Specific names are often given from some circumstance of the leaf; 

 the Hepatica triloba is that species of the Hepatica. which has leaves 

 with three divisions, called lobes. The viola rotundifolia, is a spe- 

 cies of violet with round leaves. 



A knowledge of the various appearances presented by leaves, is 

 of great importance to the botanical student; in order to become ac- 

 quainted with these, much practice in the analysis of plants is neces- 

 sary. Engravings will assist you in understanding the definitions, 

 but you must chiefly consult nature. 



Definition of the leaf— Utility of leaves to the whole plant— T'.ie period at which 

 leaves appear— Foliation— Aphyllous plants— Leaves furnish specific characters. 



