Anomalous Secondary Growth

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in this TS, near the centre of the stem, you will see some primary vascular bundles embedded in lignified pith parenchyma. Move the slide towards the outer regions, and you will notice that there has been fairly extensive production of secondary vascular tissue. Look for the vascular cambium. Secondary phloem and secondary xylem lie on either side of it. The secondary xylem is composed of tracheids,fibers and narrow-diameter vessels. Interspersed with the secondary xylem you will be able to see small pockets of phloem and what look like large-diameter metaxylemvessels. These are reminiscent of the primary bundles towards the centre of the stem. These are in fact primary vascular bundles embedded within the secondary xylem, hence the use of the term, anomalous growth in this instance. The phloem is described as being included phloem, which by definition is phloem tissue which lies between regions of secondary xylem. Whilst the physiological advantage of the formation of included phloem has not yet been studied, one could speculate that in this instance, the included phloem would be well-protected from predators and pests and, of course, be well-supplied with water and nutrient. The anomalous growth results as as a result of differential cambial activity. Newly-produced vascular cambia result in the outer lateral meristem becoming quiescent, and this cambium returns to activity only when the internal vascular cambium (which produce the individual embedded bundles) become less active. Vascular cambia are said to not produce rays in Nyctaginaceae (lateral meristems do), but do produce vessels and associated, axial parenchyma and sometimes fibres to the inside and variable secondary phloem to the outside.

Invertedmedullary bundles.At first glance, this stem looks like a typical dicot stem, which is undergoing secondary growth. A broad band of secondary xylem ( 2X)occurs exarch to the primary xylem, and this xylem terminates withprotoxylem (PX) internally. External to the xylem, one can see that the vascular cambium has formed some secondary phloem(2P).Strands of primary phloem fibres(PPF) are present. Look carefully at the tissue internal to the protoxylem. A cambial zone is evident, as are areas where internal phloem has developed. This internal cambium forms inverted medullary bundles in Campsis.The Bignoniaceae are known to exhibit several anomalies in their stem and root structure. Amongst these, the formation of a row of inverted medullary vascular bundles at the margin of the pith are common in a number of species in this family. The illustration to the left, shows a detail of one such inverted vascular bundle, which occurs just beneath the protoxylem(PX) of the normal vascular development in this stem. This inverted bundle is associated with a cambial zone(CZ) which has started to produce some secondary vascular tissue. Note the file of sieve tubes(S) to the inside of the internal vascular cambium.

Young stem at the end of primary growth. A closed ring of cambium arises between xylem and phloem. This is the beginning of (secondary) growth

Cross-section through an old stem with secondary growth. A vascular cambium has been formed which deposits xylem (to the internal side) and phloem (to the external side). The cork cambium under the epidermis forms cork to the outside.

in the woody part wide parenchyma rays provide a connection through which transport of water and nutrients remains possible.

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Anomalous Secondary Growth - Presentation Transcript

Anomalous Secondary Growth
Post lab discussion
Anomalous secondary growth
growth form which does not follow recognizable patterns that occur commonly in the majority of vascular plants
does not follow the pattern of a single vascular cambium producing xylem to the inside and phloem to the outside
Significance
taxonomic implications
physiologic adaptation
- climbing plants and lianas;
-modified organs (rhizomes, tubers, corms)- shortening of the internodes and extensive devpt of storage parenchyma
Bougainvilla(Nyctaginaceae)
Old stem
Young stem
Bignonia (Bignoniaceae)
Aristolochia
1 epidermis3 parenchyma4 cortex5 sclerenchyma7 primary phloem8 primary xylem9 pith2 collenchyma6 parenchyma
1 cork3 sclerenchyma4 phloem5 xylem7 pith2 cortex6 ray
Bauhinia – unequal activity of cambium
For flexibility