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the leaf. The axis of the embryo proceeds downward and upward 

 simultaneously, the descending axis being the root, and the ascend- 

 ing one the stem or trunk. Upon these axes others are formed as 

 subterranean or aerial branches. The leaf is formed upon the 

 ascending axis, and besides its common form, it assumes, while obey- 

 ing the same fundamental laws, certain other forms, as in the sepals, 

 the petals, the stamens and pistils, Schleiden, in ' The Plant, a 

 Biography,' gives us a picture of a typical plant constructed on this 

 principle. This makes a plant a dual, or composed of two essentially 

 different parts. 



" But to us it appears possible to reduce a plant, by a more en- 

 larged conception of its nature, to a unity. According to our idea, it 

 consists essentially of a stem sending out other stems similar to itself, 

 at certain angles, and in such a regular manner that the whole is 

 made to take a pre-determined form. The ascending axis, for 

 instance, sends out, at particular normal angles in each tree, branches 

 similar in structure to itself. These lateral branches again send out 

 branchlets of a like nature with themselves, and at much the same 

 angles. The whole tree, with its branches, thus comes to be of the 

 same general form as every individual branch, and every branch, with 

 its branchlets, comes to be a type of the whole plant in its skeleton 

 and outline. 



" Taking this idea of a plant along with us, let us now inquire 

 whether there may not be a raoqjhological analogy between the 

 stems and the ribs or veins of the leaf. As these veins are vascular 

 bundles, proceeding from the fibro-vascular bundles of the stem, they 

 may be found to obey the same laws. Physiological confirmations 

 of this presumption may be found in the following circumstances : — 

 1st. Both stem and vein are capable of becoming a spine, the stem 

 as in the thorn, the vein as in the thistle. 2nd. It is also an unsettled 

 question whether the inflorescence and seed-vessels, in many cases, 

 are formed out of metamorphosed leaves or metamorphosed branches. 

 The very fact that there is such a dispute shows that there is an ana- 

 logy between leaf and branch. 3rd. The vein of the leaf is capable 

 equally with the stem of producing a leaf-bud, as in Bryophyllum and 

 Gloxinia. 



" We begin with the examination of those plants which have a 

 fully-veined or reticulated leaf, and here we shall find a morphologi- 

 cal analogy between the leaf and the branch, and the leaf and the 

 whole plant. We are quite aware that in respect of physiological 

 development there is a wide difference between the two, but this will 



