LEAVES. 67 



the upper, smooth, jjrecn and shininp, the under surface gen- 

 erally with the ribs prominent, of n lighter green, often hairy, 

 and ^hounding in s'oinaj: or pores. The jiartieular pos-ition, 

 which a leal' assumes, is necessary to its projieily performing 

 its functions, and even to its existence ; for if a leaf bo inver- 

 ted it withers and dies. The deeper tint, of the upper surlace 

 <■'' s, is supposed lo he owing to tb.e greater compactness 



V piirenehyma. 



i4l 63. Tiic fii*st organs, that appear in dicotyledons after 

 germination, are the cotyledons themselves, or the lobes of the 

 seed, which supply the young plant witli food, till it becomes 

 furnished with orijans lt»r obtaininir it fr(;ni other sources. 

 r>( lore the cotyh'd'nns become exhausted, leaves are produced, 

 which are called Sf jn in al or seed leaves, \>hichare cai>able 

 in some degree of elaborating the sap drawn up by the roots. 

 Next coTue the Primordial leaves which seem lo poj-sess an 

 or«janiz ttion a decree ruore elevated. These lcave> often dif- 

 Ibr materially from the succeeding true leaves of tlie plant, 

 and seen) to form a kind of medium between the cotyledon and 

 true Irav( s. A similar course is followed by Monocotyledons 

 but as t! r cotyledon dovf: not arise above the earth, this organ 

 is not o'^scrved. When leaves have their origin at, or below 

 the surfart' of the earlii, seeming to come from the root, they 

 are ea"' '' 1 Icarrs^ although it is not strictly a correct 



tf-rm ; .. ■; to imply, t! at the leaves originate from the 



root, which is not the case, tl.«' root iind«»r ordinary circum- 

 f-tances not producing leaves. Leaves havirg their origin 

 on t!:e main stem, are callc<l Cavline ; those arising from 

 brai;e;.<s are distinguished \w the term, ramral ; when leaves 

 are loimd among the flowers or on flower bearing branches, 

 they aie called //ora/ leaves. 



64. The structure of the leaf demands our careful attention, 

 both from the singularh'' curious arranjrcment of its parts and 

 the maiiifL'st desli^n exhibited in fittinjr it for the various tunc- 

 tions, it is found to ptrform. Although to the coTTimon oh- 

 server the leaf appears a very, simple organ, composed of sim- 

 ple fibrous veins, and cellularsubstar.ee; yet bv the aid of I he 

 mirr.,^fnpc it is found to be one of the most complicated of the 

 \ le organs. 'I'he leaf is covered like the other parts of 



the plant by a cuticle, except such as are suhmerged in water, 

 a ' irnished with pores or stomates. These pores in most 

 p.wi,.>, are more numerous on the under, than on the upper 

 surface. In leaves vbich grow nearly perpendicularly, the 

 pores are more equally distributed on both surfaces, as in the 

 Iris. Those leaves which lie upon the water, have no j)ore8on 



