Woody Reading
what is the mechanism behind secondary plant growth?
ok, so here it goes...
we have this assignment:
How come the secondary structures (woody roots...) are cylindrical if they originated from star shaped formation of cells?
secondary growth of plants started from inside the endoderm (am i right?) and the mitotic cells (that divide to form these woody structures) follow the shape of the xylary elements (which is shaped like a star).... so how come the mature roots are cylindrical?
thank you very much for reading.....
live life....
1) Xylem is a small part of the primary root . The root itself is cylindrical in shape.
2 ) Xylem is star shaped is not to be taken too seriously . Just as woman's body is hour glass shaped is not taken too literally !!!
3 ) click on the link below to see xylem in its primary structure . Before s. growth .
http://cache.eb.com/eb/image?id=5605&rendTypeId=4
In the above diagram you will notice that CAMBIUM ( progenitor of s. xylem and s. phloem. You have referred to them as mitotic cells >) is located just below the primary phloem and just outside the protxylem ( Not labeled in the diagram as such )
Thus the cambium forms a wavy and continuous ring before the S. growth commences .
This WAVY CAMBIUM RING produces S. phloem OUTSIDE the ring and s. xylem ON THE INNER side of the ring.
The quantity of S. Xylem is much greater than the quantity of the S. phloem ( This is an important statement !)
As a result of this unequal production of Secondary phloem(outside ) and Secondary Xylem ( On the inner side ) the WAVY CAMBIUM RING now becomes CIRCULAR and remains that way for ever !!!!
Thus the so called star shaped xylem is now pushed towards the central core of the root and occupies an insignificant portion of the total area / volume . Morever it is now defunct!!
It is often full of waste materials as in Tilia section some where below .
click the link below to see an old root section and see how small area is occupied by P. Xylem .
http://www.botany.hawaii.edu/faculty/webb/BOT201/Angiosperm/MagnoliophytaLab99/RootSecGrowLarge300CropLab.jpg
http://generalhorticulture.tamu.edu/HORT604/LectureSuppl/AnatomyOrgans/AnatomyOrgans05_files/image043.jpg
http://www.eou.edu/~kantell/img0021.jpg
The root was , on the whole , cylindrical and remains that way all the way !!!
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