33.] CHAPTER II. THE TISSUES. 183 



when derived from a poljstelic stem, they are entire steles. The 

 structure of the leaf-bundles corresponds essentially with that of 

 the stem-bundles ; if the latter are concentric or collateral, etc., so 

 are the former, as a general rule ; but in the Cycads the bundles in 

 the petiole have the protoxylem central next to the bast (see p. 178) 

 instead of in the normal position which it occupies in the stem- 

 bundles ; and again, whilst the bundles in the petiole of most 

 Ferns are, like those of the stem, concentric, in the ribs of the 

 lamina they become collateral. At the same time it should be 

 pointed out that whilst the general relations of the bundle are 

 usually the same in both stem and leaf, the changed conditions 

 usually involve a somewhat different description. For instance, 

 the common conjoint bundles of the stem of a Dicotyledon are 

 collateral, the protoxylem being the most internal or central part 

 of the bundle, the protophloern the most external or peripheral 

 part ; on tracing a bundle into a leaf which is dorsiventral, and 

 lies in a nearly horizontal plane of expansion, the xylem of the 

 conjoint bundle will be found to lie towards the upper (ventral) 

 surface of the leaf, with the protoxylem uppermost, whilst the 

 phloem is directed towards the lower (dorsal) surface, with the 

 protophloem lowermost ; the description of the position and rela- 

 tions of the bundle must be in accordance with the symmetry of 

 the member of which it forms part. 



The Termination of the Vascular Bundle. The gradual thinning 

 out and termination of the vascular bundle can nowhere be more 

 satisfactorily studied than in leaves. The bundles, when traced 

 towards their ultimate ramifications, are seen to diminish in bulk 

 in consequence, partly, of a reduction in number of the constituent 

 elements, and partly also to the smaller size of the elements which 

 still remain. The mode of termination of the vascular bundles in 

 foliage-leaves is briefly as follows. In many cases the bundles 

 have only free ends, as in most Ptendophyta (e.g. Adiantum, 

 Selaginella), and generally in small reduced leaves. In others, 

 there are no free ends, but the finer branches anastomose with each 

 other to form a closed system ; this is characteristically the case 

 where the venation is parallel (e.g. Monocotyledons, see p. 55). 

 In others, again, the finer branches anastomose, forming a network 

 from the meshes of which the ultimate branches project among the 

 mesophyll-cells as free ends : this obtains generally among Dicoty- 

 ledons. The free ends of the bundles consist of one or two .rows 

 of short tracheids with close spiral markings ; no sieve- tubes can 



