130 PART II. ANATOMY AND HISTOLOGY. [ 33 



growth in length of the adjacent undifferentiated tissues; hence 

 the successive thickenings become more or less widely separated, 

 and the wall of the vessels may be torn and destroyed (Fig. 104). 



The primary bast or phloem consists essentially of jsieye-tissue 

 (p. 94) and of parenchyma. The sieve-tissue consists in_alLeases 

 mainly of sieve-tubes of simple structure (Fig. 74, p. 95), con- 

 stituting the vascular tissue of the bast, with which companion - 

 cells are associated in Angiosperms but not in Gymnosperms and 

 Pteridophyta. In some Angiosperms, particularly in the closed 

 bundles of Monocotyledons (Fig. 103), there is no bast-parenchyma, 

 the whole bast consisting of sieve-tubes and companion : cells : but 

 this tissue is generally present, and is readily distinguishable 

 from the companion-cells by the larger size of its cells. In some 

 cases (e.g. some Palms) the bast-parenchyma is to some extent 

 replaced by sclerenchymatous fibres ; otherwise the occurrence of 

 fibres in the primary bast is rare. 



The cambium is present in the collateral primaryjbundles of 

 the stem of most Gymnosperms and Dicotyledons; it is never 

 present in primary bundles of any other type of structure; nor, on 

 the other hand, is it always present in a collateral bundle (absent 

 in Equisetum, Monocotyledons, some herbaceous Dicotyledons, see 

 p. 128). It lies between the bast externally and the wood in- 

 ternally, and consists essentially of a single layer of merismatic 

 embryonic cells rich in protoplasmic contents, and with walls of 

 cellulose. In transverse section (see Fig. 100) the cells are oblong, 

 with their longer axes placed tangentially ; in longitudinal section 

 the cells are seen to be elongated and somewhat prosenchymatous, 

 like the procambiurn-cells, where they abut on the wood or on the 

 bast; but where they abut on primary medullary rays they are 

 short and parenchymatous. 



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 Pteridophyta (e.g. Adiantum, 

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

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



