MORPHOLOGY 133 



Crystals of calcium oxalate may be deposited in the thickening layers of the 

 bast fibres, or in their middle lamella. The single elements composing the sieve- 

 tubes communicate by means of terminal sieve-plates. The sieve-tubes of Conifers 

 have also sieve -pits on their radial walls, which (Fig. 141, vt) correspond in 

 position to the bordered pits of tracheids. At a certain distance from the cambium 

 the sieve-pits, both terminal and lateral, become overlaid by callus-plates. During 

 the vegetative period following their development, the sieve-tubes become empty 

 and compressed together (Fig. 140, cv). The rows of bast parenchyma cells con- 

 taining albuminous substances, which are found in some Conifers, undergo dis- 

 organisation at the same time as the adjacent sieve-tubes ; the bast parenchyma 

 cells which contain starch, on the other hand, continue living for years, and even 

 increase in size, while the sieve-tubes become disorganised. 



The elements of the phloem tissue included in the bast strands of 

 woody Dicotyledons are represented by sieve-tubes and companion 

 cells. To the parenchymatous tissues of the bast belong BAST paren- 

 chyma, bast fibres, and transitional forms between them. The bast 

 fibres, like the fibres of the wood, may occur in an un thickened form 

 as FIBROUS cells, either with or without living contents, or they may 

 be filled with starch, and finally may become septate. 



The bast parenchyma conducts and stores the carbohydrates, and 

 also takes up the by-products of metabolism, even to a greater degree 

 than the parenchymatous tissues of the wood. Just as in the case of 

 the Gymnosperms, the sieve-tubes of Dicotyledons remain functional 

 but for a short time, afterwards becoming empty and compressed. 

 The companion cells experience the same fate as their sister-cells, the 

 sieve-tubes, while the starch - containing bast parenchyma remains 

 active for many years. The different appearance presented by the 

 bast of various woody Dicotyledons is due to the larger or smaller 

 lumen of the sieve-tubes, to the presence or absence of bast fibres, and 

 also to the manner of distribution of the component elements. 



An example of bast with an especially regular arrangement is afforded by the 

 Lime (Fig. 149). In a cross-section, even under a low magnifying power, an 

 alternation of . shining white and dark - coloured tangential bands is noticeable. 

 When more highly magnified, it can be readily seen that the white bands consist of 

 strongly thickened bast fibres (Fig. 149, I). Adjoining them, there follow, towards 

 the periphery, one layer of bast parenchyma cells (p), then a zone of wide sieve- 

 tubes (v) and small companion cells (c) ; next to these come two layers of bast 

 parenchyma (p), abundantly supplied with "starch, and followed by a single 

 interrupted layer of bast parenchyma cells containing crystals (k), and finally, 

 another band of bast fibres (I). The farther removed the sieve-tubes and companion 

 cells are from the cambium, the more crushed they become, until ultimately they 

 appear as a compressed mass of cell walls without cell cavities. 



The medullary rays of the Gymnosperms (Fig. 137, ms) and woody 

 Dicotyledons (Fig. 145, pm, sm) form radial bands, composed wholly 

 or in part of parenchymatous elements. Their function is to supply 

 the cambium and wood with the products formed in the leaves 



