1839] 



FARMERS' REGISTER. 



207 



leaves of different plants, arises from the more 

 or less abundant deveiopement of the cellular 

 system. 



In order to make this matter plain, "we may 

 conceive of a leaf, as formed by the protrusion of 

 a portion of the medullary sheath of an exogen, 

 or of one or more of the vascular bundles ol' an 

 endogen, into the bark; which, at this early period, 

 consists wholly of cellular tissure, invested by a 

 cuticle. The continued extension of the fibrous 

 portion pushes outwards the fold of soft bark in 

 ■which it has insinuated itself; and at length the 

 fibres diverge from each other, in a horizontal di- 

 rection, and carrying the cellularsystem with them, 

 produce the flat and dilated figure of ihe leaf^" — 

 This, however, it must be recollected, is not ad- 

 vanced as a true account of the manner in which 

 a leaf is formed ; but only as something which we 

 may suppose to be true, lor the purpose of tracing 

 out the analogy existing between leaves, which 

 at first sight would appear to have nothing in com- 

 mon. 



Adopting this supposition, let us see how each 

 leaf would be formed. We will commence with 

 radiate-veined leaves. If the parenchyma is abun- 

 dant, so as to fill up all the spaces between the 

 veins, we have a rounded leaf, as in the ranuncu- 

 lus abortivus. Where the parenchyma is less 

 abundant; v/e have a leaf, still rounded in its gen- 

 eral shape, but lobed or toothed along its margin ; 

 as in the ranunculus recurvaius. Where the pa- 

 renchyma is still less abundant, these indentations 

 will extend nearly to the base of the leaf, and thus 

 the palmate, or hand-shaped form of that organ, 

 will be produced, as m \\\e rammculus seleratus. 

 Where the parenchyma, is only sufficient to form 

 a covering for the veins and sub-veins, we have 

 the multifid or many-parted form of the leaf; as 

 in the ranunculus fluvlatalis. Such a supposi- 

 tion as the one adopted, is not entirely gratui- 

 tous, for in many genera of plants, as in that of 

 the ranunculus, or butter-cup, from whicli the 

 above instances are taken, we notice all the inter- 

 mediate forms, between an entire and a dissected 

 leaf; indeed a large part of the series is often pre- 

 sented by the leaves of a single plant. 



If we take the feather-veined leaf, and conceive 

 the parenchyma to be so abundant, as to fill up all 

 the spaces between the veins, we shall have an 

 entire iance-shaped leaf, as in the peach, (amyg- 

 dalus persica.) If the parenchyma is less abun- 

 dant, we shall have a lobed or toothed Iance-shap- 

 ed leaf, as in the scrub-oak, (quercus catesbaei,') 

 or in the beech, (fagus sylvatica.) If we con- 

 ceive it to be sufficiently abundant, and so arran- 

 ged, as to fill up the interstices between the secon- 

 dary divisions of the vascular system, but not those 

 between the primary, we have a compound leaf, 

 in which the several leafets are arranged in lines, 

 on both sides of the mid-rib, as in the yellow lo- 

 cust, (robinia pseudacacia.") If we conceive the 

 interstices between the tertiary divisions of the 

 vascular system to be filled with parenchyma, 

 whilst those between the secondary and primary 

 divisions are not, we have the decompound leaf, 

 such as that of the honey-locust (gleditshia tria- 

 canthos.) Again, if we take a parallel-veined 

 leafj and conceive all the interstitial spaces be- 

 tween its veins to be filled up with parenchyma, 

 we shall have the linear leaf, like those of grapes. 

 If we conceive each vein to have only sufficient 



parenchyma to cover it, we shall have a leaf like 

 thai of the pitch pine (pinus rigida.') 



Without pursuing this examination further, we 

 will notice, that in every variety of leaf there i3 

 some part of the vascular system in contact with 

 every part of the cellular system. The peculiar 

 office of ihe first, is believed to be, that of convey- 

 ing liquids to and from the latter ; whilst the office 

 of^ the latter is to elaborate the sap. The whole 

 leaf may be compared to a manufacturing town ; 

 the veins being the streets, through which the raw 

 materials are brought in, and the manufactured 

 articles carried out ; whilst the rows of cellules are 

 the shops, in which the manufacture is effected. 

 Such is the arrangement, that there is no street, 

 which has not its row of shops, and no row of 

 shops which have not their street. 



h' we examine the internal structure of the leaf 

 more minutely, we will discover in every part of 

 it, an admirable adaptation to the same end. The 

 process termed elaboration, consists in the conver- 

 sion of the crude sap into the different proximate 

 principles, or "con)pound elements," of plants; 

 such as sugar, resin, &c. This process, as has 

 been already mentioned, is believed to be princi- 

 pally performed in the leaves. To its performance, 

 the evaporation of water, and the absorption ofgas- 

 eous matter from the atmosphere, appear to be in- 

 dispensibly necessary, h' with a powerful micro- 

 scope, we examine a cross section of an apple 

 leafj we will notice, along the upper and lower 

 surfaces, a thin transparent membrane ; this is the 

 cuticle, and envelopes the whole leaf. Immedi-. 

 ately below this, there are three or four strata of 

 spheroidal cellules, closely packed together, and so 

 placed as to have their greatest diameter perpen- 

 dicular to the surface of the leaf! Below these, 

 and next the lower surface, we will see three or 

 four more strata of cellules, very irregularly placed, 

 and very loosely packed together. The upper 

 part of the leaf has a compact structure; the low- 

 er, a cavernous one. And hence, in part, arises 

 the deeper color of the upper surface. When we 

 call to mind the fkct, that it is the upper surface 

 only which is exposed to the direct action of the 

 sun, we will see, in the more compact arrange- 

 ment of its cellules, a provision to prevent a Too 

 rapid evaporation of its liquids. 



The cuticle, which covers the upper surface of 

 the apple leaf, is nearly entire throughout; whilst" 

 that which covers the lower surface is pierced with 

 numerous holes. Through these openings, the 

 gaseous matter necessary for the sustenance of the 

 plant, is absorbed ; hence they have received the 

 name of breathing pores. In consequence of its 

 cavernous structure, the lower part of the leaf; 

 presents a much larger cellular surface to operate 

 upon the air, than the upper. In aquatic plants, 

 whose leaves float upon the surface of the water, 

 and of course have their under surfaces cut of!' 

 from all communication with the atmosphere, ^the 

 structure of the leaf is reversed. The breathing 

 pores are all on the upper side. In grapes and 

 other similar plants, whose leaves grow in a near- 

 ly perpendicular direction, thus exposing both sur- 

 faces alike, to the action of the sun and the atmos- 

 phere, the breathing pores are about as numerous 

 on one side as on the other. That such is the 

 fact, will be evident, fi-om the following statement 

 of their number, on the upper and lower sides re- 

 spectively, of the leaves of several plants, as given ' 

 by Prof Lindley: 



