66 BOTANY 



to lie exactly inside the bast strands, the two being 

 separated only by a layer of meristematic cells known as 

 cambium. The bundles in the shoot are known as con- 

 joint bundles from this association of the wood and bast. 

 The wood strands, further, become twisted on their long 

 axes in this same region, so that the protoxylem, which in 

 the root is on the outside, is in the stem on the inner face. 

 The bundles are wedge-shaped much as they are in the 

 root. Each seems thus to have turned completely round 

 so as to face in the opposite direction. Instead of the 

 cylinder being solid in the centre, the conjoint strands 

 always stand round its periphery so that there is a large 

 unoccupied space in the centre, known as the pith. 



Following them to the growing end of the stem we 

 find that they do not terminate in its growing cells, but 

 can be traced into the leaves through their petioles. 

 In the latter they usually form a half cylinder open on 

 the upper side, instead of a complete hollow cylinder as 

 in the stem. From the petiole they can be traced into 

 the flattened portion of the leaf, where they form the 

 network which we call the veins of the leaf. 



While the leaf and stem are very young we find in 

 them the first traces of the origination of these strands. 

 They appear in the growing point a little way back, 

 as separate strands in the plerome, made up of small 

 cells, longer than broad, defined from the rest chiefly by 

 their smaller transverse diameters. They are all meri- 

 stematic and only slowly lose the power of dividing. A 

 transverse section (Fig. 26) of the plerome shows these 

 little strands as wedge-shaped areas, the procambial 

 strands, arranged in a circle near the outside of the 

 plerome, separated by narrow areas known as medullary 

 rays. As they get older the cells become changed into 

 their adult form. The change in the wood cells is asso- 

 ciated with growth in diameter and irregular thickening 

 of the walls, making them appear as if marked out into 



