33.] CHAPTER II. THE TISSUES. 181 



longitudinal section, on account of the loose spiral or annular 

 thickening's of their walls. The looseness of the spiral or annular 

 markings is due to the fact that these vascular cells are the first 

 formed constituents of the bundles, and that consequently they 

 are considerably stretched by the continuance, for a time, of the 

 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. 138). 



The primary bast or phloem consists essentially of sieve-tissue 

 (p. 135) and of parenchyma. The sieve-tissue consists in all cases 

 mainly of sieve-tubes of simple structure (Fig. 97, p. 136), 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. 137), there is no bast- 

 parenchyma, the whole bast consisting of sieve-tubes and com- 

 panion-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 primary bundles 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 Dicoty- 

 ledons, see p. 177). 



It lies between the bast externally and the wood internally, 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. 133) the cells are oblong, with their 

 longer axes placed tangentially ; in longitudinal section (Fig. 

 153) the cells are seen to be elongated and somewhat prosenchy- 

 matous, like the procambium- cells, where they abut on the wood 

 or on the bast ; but where they abut on primary medullary rays 

 they are short and parenchymatous. 



Variations in the structure of a bundle. It is mentioned on p. 173 

 that the leaf-trace bundles of Palms thin out as they curve out- 

 wards towards the pericycle in the lower part of their course ; the 

 bundle is, in fact, thickest where it is most deeply placed in the 

 stem ; the difference in size of the bundle, as seen in trans- 



