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\\'ith leaves that strike the eye, the leaf and plant are typically analo- 

 gous. The leaf is a typical plant or branch, and every tree or branch 

 is a typical leaf. I am quite aware of the differences between these 

 two distinct members of the plant. In particular, we find in the case 

 of the full tree that the branches extend all round the axis, whereas 

 in the leaf the fibrous veins all lie in one plane. But then we have 

 a phenomenon to connect these two in the branch, the branchlets of 

 which often lie in one plane. The principal difference between the 

 tree and leaf may probably be found to be in this, — that the cellular 

 tissue or parenchyma, which in the tree and its branches is collected 

 into the pith and bark (which are connected by the medullary rays), 

 is in the leaf so spread out as to fill up the insterstices of the fibrous 

 matter which forms the veins. 



" The general order as thus stated applies only to the plants which 

 have pith and bark and fully-formed leaves, intended to strike the 

 eye. There is no such special order in plants with linear unbranched 

 leaves, such as firs and pines. The leaf in these plants has no rami- 

 fied venation, and seems to correspond, not to the whole tree, but to 

 the stem, and in doing so it is more in accordance with the whole 

 morphology of the tree than a veined leaf could possibly be. But 

 while the general order is varied to suit the different physiological 

 structure and form of tree, we discover here the very same general 

 principles of order as we have been discovering elsewhere ; for in the 

 firs and pines every internode is of the same structure with every 

 other; every branch tends to assume the outline of the whole tree; 

 every topmost or growing internode, with its leafage, is of the same 

 form as the tree or branch. Herein does the special morphology ap- 

 proach nearest to that of the plants with ramified veins ; and the very 

 cones are often types of the whole tree, and of every branch. 



" We are not prepared to say what is the special law of order in 

 plants of the monocotyledonous class. Some of these, such as our 

 ordinary lilies and grasses, send off no branches ; and the leaves of 

 these plants have their veins parallel or nearly parallel to the stem, 

 and have no ramified venation. In regard to palms, they would 

 require to be investigated in their native climes before their special 

 order could be discovered. Some plants of this class, the dictyogens 

 of Lindley, to which belong the yams, have branches like our ordi- 

 nary forest trees; and it is a curious circumstance, and confirmatory 

 of our theory, that the leaves of these plants have a reticulated 

 structure. 



" So far as Fungi, lichens, Algao, and the whole acolylcdonous 



