DEFIXITKIX AND CLASSIFICATION' OF STEMS. 649 



been used as a mark for distinguishing between leaf and stem, and the peripheral 

 tissue has been exphiined as the basis of the leaves, and the tissue lying below it 

 as that of the stem-structures. The outer layer of cells of the growing cone, called 

 de)~mato(/in, never forms the starting-point for latei"al stems, although it may 

 occasionally give rise to leaves. Tlie two or three outer laj^ers of cells of the tissue 

 below, called the periblem, usually form the leaves, but lateral stems often originate 

 from tlie second to the fourth layers of the periblem. However, these possible 

 dift'erences of origin are insignificant, and no sharp limit can be drawn between the 

 tissues from which originate the rudiments of leaves and lateral .stems; consequently 

 in this particular point there is no essential ditl'erence between leaf and stem. 



In the stem the vascular bundles form a ring round the axis, but in the leaf- 

 stalk, in other respects often very like the stem, they are grouped in a semicircle or 

 in a plane. This, however, does not invariably occur. Leaf-stalks which liear 

 peltate blades, as well as those which pass into blades with pinnate or palmate 

 strands, as, for example, those of Solanum, jasminoides, Anamirta Cocculus, Meni- 

 speiinmn Carolinianum and of many other Menispermacese exhibit cii'cles of 

 vascular bundles and an actual ring of wood, so that they cannot in their internal 

 structure be distinguished from stems. All other difi'erences between leaf and stem 

 which have been brought forward at different times and by different investigators 

 apply indeed to a number, often to a very great number of plants, but unfortunately 

 not to all. The following have been suggested as relatively the best marks of 

 distinction, viz. that the leaf shows a limited growth, and that no new leaves spring 

 directly from it, while the stem gi-ows indefinitely and produces leaves laterally 

 below its growing point. I say expressly the 7-elatively best marks of distinction, 

 because structures exist which cannot be forced into the limits of this definition. 

 The flower-bearing as vrell as the flowerless phylloclades of the Smilacinese (which 

 are really reduced axes) have always a limited growth; and, on the other hand, 

 there are plants from whose leaves other leaves grow out. In the leaf-blades of the 

 American twining plant Aristolochia Sipho, which is often met with in gardens as 

 a covering for arbours and trellis-work, green projecting bands and lobes, which can 

 indeed only be explained as leaf -structures, sometimes arise on the lower side of the 

 blade, especially in those places whei'e the finer strands form delicate anastomoses. 

 This is a case where leaf-like structui-es actually spring directly from leaves, and 

 the only difference is that the places of origin of the leaflets are not arranged in 

 geometrical succession. 



On reviewing the results of the developmental and morphological researches, 

 here only briefly touched upon, we are forced to confess that it is very difficult to 

 state aVjsolute distinctions between leaf and stem, and that, moreover, the view 

 already mentioned, viz. that the stem does not form an independent member of the 

 plant, is not really contradicted. The single fact opposed to this view is the occui-- 

 rence of stems without leaves; those, for example, which .spring from the seeds of 

 Cuscuta. But here also it may be objected that this stem in its further development 

 forms small leaves below the growing-point, and that its tissue is nothing more 



