208 



FARMERS' REGISTER. 



[No. 4 



In one square inch of the upper surface ; under do. 



Hydrangea quercifolia, (hydrangea,) none. 160,000 



Peonia, (peony,) " 13,790 



Pyrus, (pear,) " 24,000 

 Hypericum grandiflora, (St. John's 



wort, ) " 47,800 



Arum dracontium, (water turnip,) 800 16,320 



Aiismaplantago, (water plantain,) 12,000 6,000 



Iris germanica, (blue iris,) 11,572 11,572 



Dianthus cariophyllus, (June pink,) 38,500 38,500 



Tradescantia, (spider wort,) 2,000 2,000 



The leaves of the four first mentioned of these 

 plants, grow like those of the apple ; those of the 

 next two, lie partly upon the water; whilst those 

 of the last three, grow like those of grapes. 



In ihe arrangement of the fibro-vascular sys- 

 tem of the lealj we may notice an adaptation to 

 the same end, as in the cellular. The vascular 

 system, which pierces the parenchyma in every 

 direction, consists of ducts and spiral vessels, sur- 

 rounded by woody fibre. The use of the fibre is 

 evidently to give strength and support to the 

 parenchyma, and protection to the tubular vessels. 

 The bundles are largest as they enter the leaf, and 

 gradually diminish in size, towards the extremity 

 of their ramifications, till they altogether lose 

 themselves in the parenchyma. In this we notice 

 a remarkable resemblance to the circulating sys- 

 tem in animals. But are there separate sets of 

 vessels for conveying the sap outwards, and for re- 

 turning it again to the trunk ? As I have already 

 mentioned, there is believed to be. Not that we 

 are able to demonstrate the fact by actual exami- 

 nation; for to the eye, even when aided by the 

 most powerful microscopes, the smaller veins 

 seem to be altogether single. But then "it is well 

 known, that when a leaf has been macerated till 

 the parenchyma is quite decayed, the veins will 

 separate into an upper and lower stratum, so neat- 

 J}' and regularly, that it is impossible to doubt, 

 their having originally consisted of two distinct 

 systems, the one placed above the other. As the 

 sap is propelled from the wood into the leaf in a 

 crude slate, and returned into the bark in an ela- 

 borated state, and as the veins are evidently con- 

 nected with both the vvood and the bark, it is in- 

 ferred, that the upper of these layers is for the 

 outward flow of the crude sap, and the lower lay- 

 er for the backward flow of the elaborated eap." 

 As we shall have occasion, when speaking of the 

 food of plants, again to recur to the internal struc- 

 ture of the leaf, we will dismiss it for the present. 



Leaves in their position on the stem, always ob- 

 serve some particular order of arrangement; and 

 this order is uniform in the same species. The 

 most common orders of arrangement are, that in 

 which the leaves are placed alternately ; and that 

 in which they are placed opposite to each other. 

 Respecting the first mode of arrangement, we 

 may notice, that the leaves are seldom placed one 

 above another, on exactly opposite points of the 

 stem ; but usually the second leaf is a little to the 

 right or left of the point directly opposite the first ; 

 and in like manner the third, a little to the right or 

 left of the point directly opposite to the second ; so 

 that tracing along the bases of the leaves of a 

 branch, we describe a simple spiral. When the 

 leaves are quite close, the spiral arrangement is 

 very evident; when widely scattered it is less so, 

 but yet may be discovered by close examination. 

 In the apple tree the spire consists of five leaves; 

 the sixth forms the commencement of a second se- 



ries, and is placed directly above the first. This is 

 the most common number in the spirals of alter- 

 nate leaves. "The spiral arrangement of leaves 

 is a beautiliji provision lor securing the symmetry 

 of the branches; the wood being formed by the 

 action of the leaves; if these were not placed in 

 regular orderover every part of the circumference, 

 more wood would be deposited on some sides of 

 the stem than on others. So when the spire is re- 

 duced to its simplest state, that of two leaves, or 

 in other words, when the leaves are alternate, in 

 the strict sense of the term, the stem is usually 

 two-edged, or flattened from this very cause. In 

 opposiie-Ieaved plants, the pairs are placed in an 

 alternate manner with respect to each other, that 

 is, the second pair is at right-angles with the first, 

 the third pair exactly over the first, and the fourth 

 over the second. It results from this disposition, 

 that the stems of opposite-leaved plants, arc com- 

 monly more or less four-sided, instead of being 

 cylyndrical, as in alternate leaved plants." 



During that continual elevation of the sap of 

 plants, which takes place in the cellules of the 

 ieaf^ these is always more or less solid extraneous 

 matter left behind. In the structure of the leafj 

 there is no provision made for getting rid of this. 

 It of course accumulates in the cellules, clogs their 

 vital action, and renders them stiff and hard. By 

 the close of summer, the cellules become so much 

 clogged, as to be unfit for the further performance 

 of their fijnctions. In the fall of the leaf, we see 

 a wise provision of nature, for getting rid of such 

 portions of the vegetable structure, as are no lon- 

 ger fit to fill their place in the economy of vegeta- 

 ble life. Hoary headed winter removes the worn 

 out machinery, to make place for a new set, which, 

 the hand of spring shall supply. But how is the 

 fall of the leaf brought about? It is generally said 

 to be caused by its death. If this be the tact, why 

 is it, that when a tree is killed during the summer, 

 by lightning or some other violent means,the leaves 

 instead of lalling, remain attached to the stem un- 

 til they actually rot offl The true explanation of 

 the matter appears to be this. When the leaf and 

 the stem are both growing, and in a healthy state, 

 the base of the leaf and the branch which bears it, 

 increase at the same rate, and there is no interrup- 

 tion to their mutual adaptation to each other. But 

 when at length the full grown leaf looses its vigor, 

 by the deposition of extraneous matter in its sub- 

 stance, and thus becomes rigid and unyielding,' 

 its base can no longer adopt itself to the growing 

 stem. This happens at just the time at which the 

 stem is growing most rapidly, fi'om the deposition 

 of wood beneath its bark. In this way, the union 

 of the leaf with the stem is weakened, the con- 

 necting vessels are broken ofl^, and so soon as the 

 frost of autumn has finished the work, it is ready 

 to fail before the first blast. This explanation ac- 

 counts for the early fall of the lower leaves of her- 

 baceous plants ; and for the fall of the old leaves 

 of ever-greens, at the time when the wood of the 

 ensueing summer is being deposited. It also ac- 

 counts for the fact, that the leaves of endogens, a 

 class of stems which admit of no increase of diam- 

 eter after they are once litirly formed, do not fall, 

 but simply perish and decay upon the stem, as is 

 witnessed in the leaves of corn and all grasses. 



Leaves differ very much from each other in their 

 size ; some beincr nothing more than minute scales, 

 in which you can hardly detect a distinct organi- 



« 



