858 A TEXTBOOK OF THEORETICAL BOTANY 



which the strand of procambium runs, and it is isolated by a gap from the 

 older, differentiated xylem further down the stem. The procambial strand 

 itself, on the other hand, is usually continuous longitudinally. 



The subsequent tracheids of the strand are added to the first in an upward 

 direction towards the leaf primordium, and some are also added alongside 

 the first tracheid, but comparatively few below it. The gap persists for some 

 time, but is eventually closed by two downward branches of short tracheids, 

 which border the gap and continue their downward course of differentiation 

 in the internode below, alongside the xylem already there. The influence 

 of a leaf rudiment on the course of differentiation in the stem extends at least 

 beyond the first node below and may extend for four or five internodes 

 in some cases. When the leaf traces later become merged in the general 

 mass of secondary wood it is no longer possible to be certain of their identity, 

 but there is some evidence that even in the lower part of the stem they act 

 as physiological units and are influenced by the growth of the leaf they serve, 

 so long as it persists. 



The most important point is that the differentiation begins at the leaf 

 base and extends from there first upwards then downwards. 



The isolated uppermost tracheid, rich in soluble materials due to the 

 breakdown of its protoplasm, acts as an osmotic cell, drawing water upwards 

 from the xylem below^ and thus promoting growth around it. It forms the 

 focus of a growth unit which extends from the leaf base downwards to the 

 point, which may be several internodes below, at which the leaf trace is 

 linked to the vascular system. Each such region grows independently, the 

 lowest cells vacuolating first, the uppermost expanding last and continuing 

 growth longest. 



These first elements of the vascular system are generally known as the 

 protoxylem and protophloem respectively, two names which have figured 

 largely in descriptive anatomy. Their significance lies in their being the 

 starting points for xylem and phloem differentiation respectively and thus 

 determining the pattern of that development. The later-formed tissues are 

 called metaxylem and metaphloem, and attempts have been made to 

 distinguish the two stages on structural details (Fig. 844). Protoxylem 

 elements are usually narrower than those of the metaxylem and their wall 

 thickening is usually spiral or annular. They are also usually formed in 

 parts which are still actively elongating, so that they are stretched passively 

 after formation. IVIetaxylem elements are usually wider and shorter, are 

 not stretched after their formation and are mostly pitted or scalariform. But 

 none of these distinctions is absolute or complete, and it is not possible to 

 separate clearly the two sorts of xylem tissue. This applies with even more 

 force to the phloem, where there is no histological distinction between 

 protophloem and metaphloem. We are only justified in keeping the two 

 sets of terms to mark successive stages in ontogenetic development, and it 

 has been well said that " the point about protoxylem is where it begins, 

 not where it ends." 



The earliest xylem elements are subject to a varying amount of stretching 



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