LEAVES. 



48 



the axils of leaves. One leaf only arises from the same organic 

 point ; a tuft oy fascicle of leaves is composed of the leaves of 

 an axillary branch so short that the bases of the leaves are in 

 contact ; that is, the foliage develops without any elongation 

 of the axis or the internodes. The complete leaf consists of 

 the lamina {blade or liml))^ its petiole and pair of stijyules at its 

 base. Sometimes there is little expansion of this organ, bnt it 

 is stalh-lihe I — generally the leaf is expanded horizontally, so 

 as to present the under surface to the ground, the upper to the 

 sky. On examining the cellular texture with a microscoj)e, it 

 is found that there is in each leaf an upper and an under 

 stratum of differently arranged cells ; the upper stijatum has its 

 cells compact, and so arranged as to be least affected by the di- 

 rect rays of the sun, while the under stratum has a cellular 

 arragement more favorable for evaporation or exhalation. 



50. The leaf is an expansion of the fibers of the bark, con- 

 nected by cellular tissue^ developed in a symmetrical manner, 

 as lateral appendages to the stem, and having a connection with 

 its internal part. It is covered with a green coat, or skin, 

 called the civticle. Leaves are furnished with pores called 

 stomata^ for exhaling and inhaling gases. They present to the 

 air a more extended surface than all the other vegetable organs, 

 and are of great importance to the vitality of the plant, by im- 

 bibing suitable nourishment, and throwing off such gases as 

 would be useless or injurious. In other words, leaves are 

 organs of digestion and respiration. 



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

 of life which it contains 13 protected tlirotigli the cold and dampness of winter. In 

 the spring, when the sun having recrossed the equator is advancing, toward 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. 



51. The arrangement of 

 the leaf in the bud is call- 

 ed vernation {per., spring), 

 prefoliation {p7'C6^ be- 

 fore), and gemmation f}. 

 {gemma., a bud) ; it dif- 

 fers in different species, 

 but in each follows 

 regular law. 



Figure 37, at a, shows a young 

 leaf of the currant ; this is fold- 

 ed. At 6, is a young leaf of the 



monk's-hood ; this is inflected. At c, is the young leaf of a fern {Aspidium) 

 circinate, or rolled from the summit toward the base. 



Fig. 37. 



Origin of leaves— Complete leaf— Different strata of the leaf.— 50. Structure of the leaf— Utility 

 of leaves to the plant — a. Expanding of the bud. — 51. Arrangement of the leaf in the bud, Fig. 37. 



