About casparian strip (A-level)

There are 2 reasons to explain your logical question.

理由1:
First, study the water flow carefully.
Water由soil particles去epiderms去cortex去endodermis去pericycle
去xylem

而在endodermis的endodermal cells中有casparian strip出現,即是水不可
以用appoplast去傳送了. 水只可以由passage cells及endodermal cells
without extra-thickening去傳送了. casparian strip就好像一個關了的閘,水不
能經由endodermal cells中的casparian strip入去xylem或由xylem流出,所以
水cannot diffuse back to the cortex
注意:應該是入或出也不可經casparian strip而不是”只有入經casparian
strip之後而無得經casparian strip出”.

理由2:由於mineral salts are continuously secreted into the xylem by ACTIVE
TRANSPORT, the osmotic potential of water in xylem should be very low.
This low osmotic potential prevents water from being withdrawn from xylem back to cortex.

2008-06-13 14:26:31 補充：
Correction:
水只可以由passage cells及endodermal cells without extra-thickening去傳送了
改為水只可以由passage cells without extra-thickening及plamsodesmata去傳送了

The casparian strip is impremeable to water and mineral salt, thus, water and mineral salt cannnot flow through it.
the water and mineral salt entered the stele through the endodermis.